I have a 94 firebird 7.5 10 bolt so is 35 thousandths a good shim for a 4 10 pinion?

It's not so much about what ratio you are installing, it's more about the checking distance (cd) marked on the pinion. Install the new pinion with the right shims to put it in the same location as the original, then use side shims with the carrier to achieve the correct backlash. Sometimes you can reuse the original shims, sometimes you need different ones.