fwiw, i have the same trans as you in my sbc swapped coupe, and it is rather hard to come up with a useable gear for a nissan diff. i had considered going to a solid rear axle with something like a 3:08, but thats a little too low for me really. unless you have an e overdrive youll find the same problem.
im actually going to go to a 5 speed so i can have an overdrive gear and keep a decent rear end gear, and not have to spin to engine way out to get any kind of speed.
http://www.f-body.org/gears/
this helped me a lot. good luck!