Besides the number of gear teeth and overall diameter, there are two types of flex plates. One flex plate has a "flat" crank flange bolt circle with a spacer between the crank flange and the flex plate, and it uses longer bolts. The other has a "dished" crank flange bolt circle. Just something to think about because I have see this happen before.
Spacer, GM 12563532 (1 spacer)
Bolts, GM 11569956 (pack of 10 bolts) *
*Some dealers will break up a package of parts and some won't.