My 85 truck has 305 with 8.5 rear. The 3.73 gears are right, lockup 350C trans is 2800 rpm at 65mph, cruise control on. Feels busy but very cruiseable. Trucks can have long 1 peice driveshafts. Use the OEM aluminum shaft from a 03-05 envoy. They are 9lb 66" 4.5" diameter peices that shave some weight, find them for 50-60 at parts-car.com . The OE cast shaft is a twig and may have a vibration at 3000 rpm cruise. Aluminum is smooth at that cruise. After doing the AL pullies, shaft, and some other rotating wt shaving, you will find a difference in power and mpg.
After switch gears use TCI $45 governor kit to reprogram shift points out to at least 5k rpm.
The BM shift kit for 30 has a HD setting that will increase drive pressure, shifts quick not harsh. That gold modulator spring in kit, gets you back to first from higher speeds, for a knock out kick down passing. Also for fish tailing downshifts around the corners.
Mobil 1 ATF is a good multipurpose fluid.
If you like footbrake driving or slug starts go 2600 stall. Adds a nice dimmension to the launch. Wind it up and let go.