Thanks, it's been an interesting project. As for the gearing, the disc brake diffs I've run into from the IROC'S/TransAms have been either 3.08 or 3.42. I'm now running a 3.42 which lets me cruise at 1900 rpm with a 29" tire at 65 mph.
Your 3 speed, 1:1 in 3rd gear, and let's say, you were running a 28" tall tire your engine would be turning 2668 at 65mph. If your diff was a 3.08 you'd be turning 2402. I've found that the 235 will lug down to 1000 rpms in my 4th gear (1:1) and have decent pickup with the 3.42. It's happiest between 1800-2500 rpm's but will easily scoot up to 3500 when pushed.
What you've got to do is pull the cover and determine exactly what ratio you've got. Then determine what kind of driving you're going to do. I'm including a link so you can plug in some numbers to determine your ratio/rpm's:
http://www.csgnetwork.com/multirpmcalc.html If it's a 3.08 with the 28" tire you'd be turning 2402 at 65mph. If you went with a 5 speed behind your 235 you'd be safe with the 3.42. The 3.08 wouldn't work with a .72 5 speed overdrive and the 235. Can't give you any direction with the GM V8or the pricey 6 speeds.
As for the brakes, I first upgraded to a dual drum/drum master cylinder and all new drum system. Performance was marginal in traffic. Front discs made it good, 4 wheel discs make it great! However, the front disc conversion, dual master, bracket (if you have to buy one), residual valves, proportioning valve and all the fittings and lines are expensive. Setting up your system to function properly is NOT a weekend project! Discs on just the rear aren't going to give you much of any improvement without discs on the front. In fact, I don't know exactly how you would plumb them in to keep them from locking up.
I'd suggest you use the "search" feature here on the sight to help you put together a "plan of attack". You might be better served to keep your current diff in place until you can go after the total brake conversion. I hope this helps.
