Decision: 5x4 (SBP) or 5x4.5?
So first off, if you're debating SBP vs BBP and want disks, I assume you have drums. Which honestly means you already won't be original either way, even if you add SBP disks. It's a pretty small point to argue that you still have the SBP if you've already upgraded to disks.
Question 1:
The only advantage to keeping the SBP is originality. It has no other advantages IMO. It makes wheel selection difficult for anything other than original reproduction rims, and that limits you to 14" and 15" rims in a very small selection of styles and backspaces. That might not be a problem for you though depending on your plans for the car. If you're keeping 15" rims and narrow tires it doesn't matter much. If you want to use tires that actually grip and go to 17's or 18's SBP is a non-starter.
The advantages to going to BBP are numerous. The biggest thing for me isn't even the wheels, it's the brakes. Brake parts for the original SBP brakes are not always easy to find. Replacement drums weren't even made for awhile. Hubs are not reproduced, so if you damage a hub you have to find a good original replacement. Pads and shoes are available, but in limited compounds. The original KH calipers, while a multi-piston design, are known to have issues with sticking and seal maintenance. And like everything else they can be expensive and hard to find. The 73+ BBP calipers are readily available, they are reproduced and can be found original, rebuilt, or brand new reproduction. The large single piston is virtually maintenance free. Pads and shoes are available in more compounds, the larger wheel bearings are more common to find and allegedly last longer, the larger ball joints are stronger and won't require an adaptor. The only disadvantage is the wider track width, but really that's not an issue because most wheels for BBP are available with enough backspacing. For a frequent driver the BBP brakes are easy to find parts for and easy to maintain. And that's a big advantage.
Question 2:
The piratejack kit is a 73+ disk brake set up that uses a BBP rotor blank. It is then drilled for SBP and the hub is turned down to fit with smaller wheel hub bores that the original SBP wheels have. It uses 73+ spindles, so it uses the large upper ball joint which means either changing the UCA's or using an adaptor, just like a 73+ disk swap. It widens the track width the same amount, because it
is a 73+ BBP disk set up, just with the small bolt pattern. So, advantage wise, it uses all the same parts as a 73+ disk brake set up except for the rotor, which is just re-drilled and turned down at the hub. It's the same swap except for the bolt pattern on the rotor.
However, the hub can only be turned down so far, and like some other aftermarket disk kits (wilwood) the hub is still too large to fit most original SBP wheels. PirateJack advertises that SBP drum wheels may not fit, and there have been members here that couldn't fit original SBP wheels. Wilwood is even worse, the hub is a larger diameter than the piratejack hubs. That means that your wheel selection is even MORE limited, because you have to find a SBP wheel with a large hub bore. Also the wider track is an issue too because you basically need a SBP wheel that has a BBP wheel backspace. Finally, the piratejack kit has had some production issues with the rotors. There's a big thread on the kit somewhere, I'll try to find it and link it. The brakes themselves work well, but honestly you should just go BBP if you want to use the piratejack kit. There's no reason to keep the SBP if you've gone to that kit. Originality is already out the window, and keeping the SBP adds even more wheel selection issues than if you used original SBP disks.
Question 3:
4 wheel disks really aren't necessary. Yes, they do improve stopping slightly. They're usually lighter, and you have the standard disk vs drum advantages - they adjust automatically, they work when they're wet, and they don't heat fade as fast. But if you've stayed with 15" or smaller diameter wheels you can't run a grippy enough street tire to need the improved stopping power. And if you're not driving like an animal at high speeds over long periods the rear drums don't typically fade much. If your road racing it might come up, but even autoX and spirited mountain roads won't typically be enough. The maintenance on the disks is easier, but rear drums last twice as long as the disks up front so maintenance isn't very frequent anyway.
Bottom line is the rear disks look cool, but aren't a very good "bang for your buck" upgrade. DoctorDiff is a great source if you want rear disks, his kits allow the use of the original tapered wheel bearings. Almost all others require the use of the "green" style bearings. Both work fine, I like the original tapered bearings. I run DoctorDiff's 11.7" rear disk kit on my Duster, it works great but the difference compared to the 11x2.5" drums I was running before is fairly small.
DoctorDiff is a great source for brakes for these cars, he carries everything you need for a BBP swap in the front and the rear. He has original style 73+ conversions, as well as upgraded options with larger diameter rotors. The factory offered 11.75" front disks on the B/R bodies that use all the same parts as the 73+ disks except for the rotors and caliper brackets, and that makes for a great upgrade in braking even compared to the 10.87" 73+ rotors. The 11.75's need a 15" wheel though.
Wilwoods..the hubs come set up for smallbolt or large bolt pattern well worth the money spent,and they go on real easy no need to change the upper control arm like you have to do with the stock '73 and up crap!!!
They don't go on half as easy as the original 73+ style brakes, you have to safety wire everything together which is a PITA. And if you do it wrong the hub can come apart. You don't have to change the UCA's for the 73+ brakes either, you can use a tapered adaptor if you like. Which is far better than using a 9" drum spindle with a design that even Ma Mopar didn't stand behind later. And the redesigned hub on the new Wilwood kit has a huge diameter, they don't fit any of the original wheels and not even some of the limited aftermarket wheels.
I recently pulled a set of wilwoods off a buddy of mine's Belvedere. Talk about a hot mess. Both calipers had two frozen pistons (one whole side on both). On one side a stud had broken, and because it broke behind the threads the safety wire did not hold it in place and it was backed out and rubbing on the caliper bracket. The previous owner had let the car sit which probably caused the sticking issue, so to be fair it was a maintenance issue. But I've never had an issue with the 73+ style disks. I've run calipers I pulled out of cars in the junk yard that had brown goo for fluid, a quick flush and everything worked great.
Wilwood's are for guys that like saying they spent the most money but aren't smart enough to know what they're actually getting.
