Math. Help me out

Okay so i know theres a way to find this out. If you know it let me know.

Im gonna be running a 4.3 in my 85 toyota, and im trying to consider what transmission to get, the th700r4 with overdrive or the th350.

Im running 5.71 gears in the back and 42" tsls (new). Is there any kind of equation i can use to figure out how many rpms id be turning if i used the th350 at 70?

I know you have to know all the specs of the th350 which i dont know, IM just hoping somebody has done this before.

Thanks.