no worries

most, but not all, dohc sierra's from 90 - 92 have a 7.5 inch rear diff
you will have to swap over your exhaust hanger bracket though from the passenger side rear because they don't have them (should be able to unbolt it and refit)

the measurements you want are to be fond on the top of the diff though, so unles you can reach it's going ot be difficult as the bottoms are the same size

if you do find a beam out the measurement at the top of the diff is 193mm of something, the smaller ones are 177 i think (this is from memory as i can't rememebr where i wrote them down)