With a Barracuda or Duster/Demon/Dart Sport you can usually fit 275/60/15's without the 1/2" spring offset kit. The other thing is, that with a 15x8" and a 4.5" backspace using BBP axles you won't have enough backspace to make use of the 1/2" offset. A factory A-body 8 3/4 with BBP axles will put the wheel mounting surface about 6" from the springs with drum brakes, you can add another 1/4" to that with disks. So a 275/60/15 is about 11" wide at the section, a 15x8 is 9" wide outside to outside lip. So 4.5" of backspace, plus 1" of tire overhang is 5.5", which leaves you ~1/2" of clearance with no offset kit and drums. With disks in the back it will leave almost 3/4", which also means the outside of the tire will be really close, if not rubbing on the inner quarter lip depending on your car and its ride height.
With an A-body rear with BBP rear axles and rear disks I'd be inclined to run at least a 4.75" and probably more like a 5" backspace for 275's. If you're going to run a 1/2" offset kit, make use of it and run at least a 5.25" backspace. If you get too close the the springs you can add a small spacer. If you get too close to the quarters, the options to fix it are a lot more expensive.
Those Wilwoods have had the same basic design for well over 30 years now, they're hardly new. Plus, they don't work any better. They're basically the same idea as the SBP KH calipers, and Ma Mopar replaced those with better in 1973. They also limit your wheel choices, as the have a very large diameter hub. Shouldn't be an issue with the Ansens, but they're pretty limiting otherwise. And then there's the aluminum hub that has to be safety wired together. There are much cheaper options that work better (like the 73+ factory brakes). But if you want bling, Wilwoods are the chrome that won't get you home. With the 73+ disks you can run a 15x7 with a 4.25" backspace and 225/60/15's, even with a 4" backspace you should be able to pull off 215/60/15's.
You're not really making sense here. I don't just run OEM brakes anymore, I run DoctorDiff's 13" Cobra style kit. Heck you've got another member with Wilwoods telling you not to get them. The reason you're getting comparisons and suggestions to go OEM is because the OEM brakes will outperform the Wilwood kit you're looking at and still fit 15" wheels. If you want to go aftermarket then there's plenty of options that will flog the wilwood kit you're looking at, but most of them won't fit with 15" rims.
Also, you can get decent pads for the OEM slider style calipers. Firm Feel has options, you can get the whole range of pads from EBC for the stock slider calipers, and Bergman AutoCraft is taking interest in carbon/kevlar pads for the stock calipers too. But if you're just driving on the street a decent set of semi-metallics will be more than fine for you. And since you say this is only a cruiser, I'm not sure I see why you need something other than regular old brake pads.
Uh, to stop the car better? With the better tire compounds that you can get for larger wheel diameters, not to mention wider tires, even A-bodies can make good use of high performance brakes.
