You say you don’t have the skills to rebuild it yourself, but it might be worth your time to at least pull the head and do some quick measuring of taper. There a tech tip article here. Super easy to do.

It’s possible your cylinders are in reasonable enough shape to get by with a hone, thus avoiding the nightmare of locating GMC pistons. Pistons could be knurled to take up some excess wall clearance as well.

Not sure how stuck you are on using the engines you now have, but might keep an eye on Marketplace. GMC sixes do occasionally show up for sale. Buying a decent running engine is likely to be much cheaper and less headache than a rebuild.

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