Just a SWAG, but I believe the GMC is a bit heavier. However, the better gas flow through the GMC cylinder head would probably make the added HP and/or torque gain of the GMC over the Chevy offset the weight advantage of running the Chevy. Chevy ports suck like a Hoover upright vacuum cleaner, with no practical way to improve the flow. There are still a few custom-cast 12 port heads around for the Jimmy engine, if your last name happens to be Rockefeller, Vanderbilt, etc. and you can afford to buy one.
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!