With it being a '70 truck, you may have the 4.11 gears already. Otherwise, you more than likely have the 4.56.
The 4.11 would be fine. Your RPM will still wrap up more than most folkd like though.
Being you have the big block, I'd suggest a 3.73 or 3.42 rearend. The only way you will get that is to swap to a 73 or newer 14 B rearend.
