Sounds to me like you've got a 26 spline .63 OD Camaro/Firebird box with an S10 tailshaft housing. If your run your numbers using the bottom calculator you'll get some idea of what you've got to work with:
http://csgnetwork.com/multirpmcalc.htmlYour problem sounds about one I had when I coupled up a 14 spline S10 .72 OD T5 with a 3.08 Camaro/Firebird diff in my 54 truck. Worked great in 1-4 as a 4 speed but was way on the short side of the torque curve to maintain 5th gear. I went to a 3.42 with a 28" tire and it pulls fine down to 45 mph in 5th gear. Cruise is 70 mph at 2000 rpms with plenty of throttle left at 80 mph. The torque curve in a 235 picks up at 1500 rpms and peaks out at about 2400 rpms. The engine vacuum peaks at 2200 rpms and you want to have your gearing so that your normal driving 55-65 mph is at 1900-2000 rpms. This gives you the ability to drop down to 40-45mph in 5th gear without having to downshift or lugging the engine.
I'm currently building up a streetrod/cruiser using the .63 Camaro/Firebird hybrid transmission to go behind a built SBC and I'm set up with a 3.73 R&P with a 28" rear tire. Just have a feeling that your stock 3.90 diff would be more in the ball park for what you've got now.
Dumping you exhaust out some shorty exhausts is going to gut your low-end torque to boot. Adding carbs is not going to help you.......run your .63 OD ratio, tire diameter, cruise speed, and R&P and jockey your R&P numbers until you get your favorite cruise speed at 1900-2000 rpms and you'll know what R&P ratio will work best for you. Hope this helps.
Dave
