You should consider all aspects of the build before you start anything.
A BBC is a good power plant. Starting with a big block, it would be wise to run a turbo 400 or 700r4 trans. That also means you need to match the rear gear for the trans you want to run.
The rear should be at least a 8.5 which shouldnt be to tough to get.
ratio once again is dependant on: trans type/ final drive ratio, rear tire diameter, cruise rpm of engine and top engine speed.
Ideally, you want the engine spinning at around 2200-2600 rpms at highway speed, say like 65 MPH.
So sit back , crunch a few numbers and ask a few more questions.
Getting the tire size/ diametr figured first would be the place to start as the end result of your build should be proper stance and appearance of tire size.
You can tailor the rear gear ratio and the trans top gear ratio from there to get the desired cruise rpm for the engine figured correctly.
You dont want your engine to be turning 1500 rpms at 95 miles an hour, nor do you want it to turn 6000 rpm at 80 miles an hour.
Fact is stranger than Fiction