You will want to look for a 82-87 T5 from an S10 then for a NWC with mech speedo.
In my opinion, you pick tire sizes based on what looks right for the vehicle, and then gear it accordingly. But I wouldn't worry too much about it, downshifting to 4th even on the interstate with that 3:36 shouldn't be too bad. Run the numbers through one of those gear calculators and see what you get. 4th would be the same as what you have now actually, since it is 1:1.
