Slant 6 to 340 swap
Hey I really need some help.
I have a 74 dart sport i'm converting to a 340 car. I have already got the motor mounts i need in order to swap out without changing the k memeber, i have the 727 tranny and the bigger 8 3/4 rear end. I am rebuilding the rear end now and the question i have about this rear end is that i was told i had to change the axles from a 5x4 bolt pattern to the big bolt pattern of a 5x4.5. obviously this will make it a little stronger, but i dont understand why i have to swap. If i put in the bigger bolt axles in the bigger rear end then i am just looking for more change in wheels and drums and brakes etc. I just need some direction in this it would be very much appreciated. do i have to change to bigger bolt pattern or will i be safe with the one thats already in it? Thanks! :)
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First I think it's great that you are doing your own work.
You do not HAVE to swap to the big bolt pattern right away. All 340 a-body cars were built with the 4" bolt pattern rear end from 68 through 72. (as were the 383 and 440 factory cars) The wheel selection is MUCH better with the BBP though.
You have a few options here.
1. You could stay with the rear end you have and run the car with it. If you have a 4" bolt pattern on the front, everything would match, if it's BBP in the front, you would have 2 different bolt patterns on the same car. (BTDT) If your car really is 4" pattern on the front, you probably need to upgrade your brakes to be safe.
2. You could sell your SBP a-body rear and use the money to obtain an 8-1/4' a-body rear or another 8-3/4 w/ the BBP. The 8-1/4 rear was used on 340 cars starting in 73.My friend has an original 73 Dart Sport, 340 4 speed w/ the 8-1/4.
3. You could convert your present rear end to BBP by getting brand new axles for about $300 but you will have to convert to BBP brakes in the rear. It looks like most of the common new axles require the green bearings unless you specifically request the tapered bearing style that the factory used.
4. You could source 70's era C body or truck 8-3/4" axles and have them shortened to fit your a-body width rear and change the brakes. IIRC, The source axles must have at least 28" straight length from the brake drum flange to the area on the axle where it starts to neck down.
I did option 4 and this is what was involved:
Bought a pair of early 70's C-body axles (deal at $35/pr) and sent them UPS to Moser Engineering in Indiana. They called me the very next morning (!) to confirm what I needed before they started cutting. Including shipping it only took about 4 days and about $110. I told them what brakes I was using and that they were going into an a-body width 8-3/4 rear. I had already sourced new backing plates ($20/pr), and 10 x 2-1/2 drums from various mopar swap meets. Bought new hardware and wheel cylinders from Rockauto. Using the big axles results in the rear track being 3/8" wider per side for a total increase of 3/4". Seeing the old SBP side-by-side with the larger axles is pretty impressive. Not only is the axle itself bigger, but the wheel studs went from 7/16" diameter to 1/2" on a 4-1/2" bolt circle.
So I probably spent about $200 for the rear BBP conversion by careful shopping and now have Road Runner sized brakes on my a-body with the 73-76 a-body front discs, adj prop valve, and run them w/o power. Another expense was different wheels. Personally, I think using 11" rear drum brakes is a waste on an a-body as they aren't needed and the extra inertial weight only makes the car slower.
