With that large piece of welded-in channel iron bracing the center of the crossmember, you won't weaken it by enlarging the holes under the bolts enough to get a universal socket on the nuts. Instead of a squared-off bottom to the holes, try making them semi-circular with a carbide burr on a die grinder, or maybe a small hole saw so you can access the nuts with a socket.

GMC engines are about 2" longer than a Chevy, so the motor mount has to be moved forward.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!