I assume you are using the same distributor and wires on all three engines? Could it be from preignition? Check the valve seat area and combustion chamber of the head for any sharp edges that may be heating up. If you go for a forth engine, try a cooler sparkplug and a 180 or 160 thermostat. See if it will live a bit longer. I find it hard to believe the piston right in front of the water pump is the hottest one, should be the farthest from it you would think! What about a slightly bent connecting rod. Just where on the piston is the scoring or rubbing? With 12 O-clock towards the fan, 1, 2, 3, etc...
If the rod is common on all three engines, I would really check it over or replace it.
These are just some ideas I had and most of them should of been eliminated by changing blocks and internals.
The 250 and 230 have a weak spot on the number one cylinder where the drivers side front headbolt is. They are prone to crack from stress.
Good luck and keep us posted, Joe.
