It's probably a 3/8-16 with some rust and crud in the threads. That's what a 216 , 235, and 261 all have, and I'm pretty sure those balancers will interchange with some GMC's. Try using a thread chaser to clean up the threads, but don't force it. I like to take a tap that's too dull to cut threads any more and grind the sharp points off the threads to make a chaser. I pulled a 216 balancer yesterday- - - -3/8-16. Chevy V8's use 3/8-24 threads in the 3 puller holes.
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
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