The CPP 5 lug kit uses GM 11" rotors and GM calipers and they work fine with the 15" rims. An advantage of using the 5 on 4-3/4 setup is that you can use the inexpensive over-the-counter GM rotor/hub/bearing/seal combination instead of the pricey "mail order only" upgrade bearings used in the 6 lug stock hub upgrades. CPP sells a bracket kit that comes with the necessary mounting hardware and special bearing spacers and you buy the rest of the components from your favorite local parts supplier. I'm running drums on the rear with a manual Corvette master cylinder and adjustable proportioning valve. I've found that power assist masters without an ABS system can get you in deep trouble during high-speed panic stops. The downside of the 5 on 4-3/4" conversion is that you are going to have to buy rims to get the 4" backspacing. No j-yard stuff that I could find. However, the selection of 5 lug 15" rims seems to be greater than for 6 lug 15" rims.
Depending on your 5th gear OD ratio, your rear tire diameter I'd use the 3.42 as the low-end R&P ratio. I originally went with a 3.08 IROC-Z posi/disc but it was just too tall. My 54, which has had all the suspension upgrades but still running a straight axle, is most comfortable at the 60-70 mph cruise speed and the 3.08 had me lugging down the engine out of the torque curve at 60 mph. With the 3.42, .72 and 29" tire I cruise at 65-70 mph at 1900-2000 rpms. I can drop down to 45 in 5th and accelerate back up to traffic speeds without a downshift.
Probably the best part of running this ratio combo is being able to get into 4th gear around town from 35 mph up and yet be able to run comfortably in 4th at 60-65 mph. This would make hilly country a non-issue. You could run the numbers for a 3.73. I haven't run into any GM 3.50 R&P's and even the 3.73's are few and far between. The 3.42's are found behind the 2.8's V6's in late 80's Camaros with automatic transmissions. Most of these diffs show very little wear. Hope this helps.
Dave
