Dale, I think I know what your problem is, almost exactly the same thing happened to my 248 once apon a time. About 30 years ago, I put a supercharger on my 248. I ran it around town for a while and it seemed fine. So I thought that it was time for a real run up to my cabin, about 100 miles away. Boy!, did that thing run!! But by the time I rolled in the drive, something was not quite right. By the time I managed to get back home, things were diffinatly not right. I pulled the head, partly melted pistons, holes in pistons and tops of pistons broken off the skirts up in the cumbustion chambers. The stock type piston certainly were not up to the job, but there was more going on. That was the end of the supercharging expierment, I just did'ent have the money or knowhow to make it work.
Over the years of expiermenting on my engines I have determined that at high levels of power, not sure exactly where, the str8 develops thermal distribution problems. The spark plug is way over on one side, actually over the deck, a really bad location, this makes the plug side of the piston much hotter than the far side. If there is a big dome in the way, it gets worse. There could be 2-300 degrees difference between sides. The piston can't take it. The only solution that I can come up with is to install oil squirters like the chevy LS engines have, aimed at the underside of the piston right at the hottest spot, and give it a good bath. And keep the oil cool. The next problem is that the front of the engine is always cooler than the back. The out put of the water pump must go into the engine at the center frost plug hole, then depending upon which block you are using, plug the coolant holes or drill new holes in the deck in order to warm the front and cool the rear. And, yes, there is a reversion problem with these engines. If the intake is too big or there is too much cam it's almost a sure bet. The way you can tell is if you are getting fuel stand-off above the carb/carbs or to a lessor degree, carbon tracking in the intake. Not a lot of fix other than less intake or cam. Have fun! alleycat