Unless you have experience in gear set up and the tools to do it, you should pull the center section and take it to a shop to have it set up properly.

 

Atleast w/ an automatic transmission, the problem is only having three gears. The nice thing about the 4 speed/overdrive (automatic) tranny is that you can run the same gears and have the best of both worlds, a good get-away and run highway speeds at a comfortable rpm.
One of the posts read; 75 mph @ 3500rpm. W/all due respect, 3500 IMO isn't a friendly rpm f/ a small block Chevy over a long period on the highway (especially a stock one). I'd think something below 3000rpm would be more in the ball park.
So, if it's an automatic, I'd make a tranny swap to a 700r4 or modified 200r4 and leave the rear end alone. The 200r4 being the exact length as the TH350 and all original components can be used (drive shaft, etc.). These 200r4s have panned out to be a good tranny f/ quite awhile now.