I'm not sure about the GMC, but the puller holes ov the Chevy balancers on the 216/235/261 engines have 3/8-16 threads. Try chucking a bronze rifle cleaning brush of about .45 or .50 caliber into a variable-speed cordless drill motor and run it at slow to medium speed with plenty of penetrating oil like PB Blaster or Kroil to clean up the threads. You might need to run a tap or a thread chaser into the holes to get bolts to engage properly.

Once the puller is attached, be sure you're pushing against the end of the crankshaft. I like to use a deep socket that will slip into the front of the balancer for the puller jackscrew to push against.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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