That's obviously a GMC engine. I don't know if a counterweight from a Chevy engine would work or not, but it's probably going to be easier to find a Chevy manifold and rob the counterweight from it. I've got both types- - - -let me dig a couple of manifolds out of storage and compare the weights. Drop me a PM at your convenience and I'll share my email address with you.
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!