There's a few ways to figure this out:

1.Dave's plumb bob method is good.
2.Mount a wheel (no tire) up, support car by suspension, break out the measuring tape.
3.Wait for guys here to chime in with what they've done, which may or may not work for you.
4. Get specs for the wheels & tires you like; for tires, section width, OD and rim widths recommended and used for measuring.

'68-'70 B-body rears typically use a 4.25-4.5" BS up to 8.5" wide, after that 5.0-5.5" gets a 10" rim in there. Usually 29" is about max OD, max section width that fits is around 12.5-13.0"...BUT as stated cars vary, including side-to-side.