oldburbs,
Looking at your picture 19 of 28 titled "oops! wrong water pump", I think, answers your question.
The large rubber cushion between the crossmember and the front saddle is not present with either a Chevy or GMC when using the original front saddle with the original crossmember. They both use a flat rubber pad about 1/4" thick.
There is also an extra hole drilled in the center of the crossmember that the cushion and saddle bolts to. Unmodified GMC crossmembers have just a pair of holes side by side, none in the center.
It looks like your truck has already had the Chevy front clip adapted to the GMC crossmember.
