If you have a 1991 C30 then you'll be replacing a lot more than the engine if you're putting in a C32 block.
Clutch, tranny, ecu, etc. The C32 isn't plug and play.
If you or your mechanic haven't planned on any of these expenditures either the new motor is a C30 w/ custom powder-coated covers or you are in for a real surprise when you go to hook up the tranny.
Just look for the engine VIN and pray it says NA1.
G