another overheating problem

'No retorque' means you don't HAVE to retorque them, but it should not hurt to do so.

The fact that you had coolant leakage that stopped after the re-torque says there was something wrong in the initial head installation. Did you use a known-good torque wrench, and did you follow the proper bolt torquing sequence? Also, you should torque the head bolts in the proper sequence in 2-3 steps to final torque. In other words, go around the torquing pattern 3 times, up to 25 ft-lbs on the first round, then 45 ft lbs on the 2nd round, then the final torque on the last round.

Are the head and block surface clean and rust free?

Inspect the head bolts and compare them side-by-side for length and look for any one(s) that are thinned in the shank or are a lot longer than the others. You're just doing a basic check for a stretched bolt.

What did you use as a straight edge for checking the head, and did you flip it to the other side and re-check? (Just to try to find any bow in your straight edge....)

The exhaust gasket leak should not have caused this overheating problem.

I would not suggest doing the piston rings; there is more to that process than just changing them out. The cylinder need honing and any ridge removed.

Can you better describe the 'gunk' around the spark plugs? Where is it? Color? Does it look like anything?