I have heard of a lot of engines that were recently rebuilt that over heated because there wasn't enough clearance between the piston and the cylinder. It seems like no one measures any more. They just stuff the pistons in the holes and consider that a .030 over piston is gonna have to work in a .030 hole.
I haven't worked on a Chev in years, just GMC. But I assume they have a petcock right by the steering box like a GMC. I have pretty good luck cleaning blocks by turning them upside down and screwing a pressure washer fitting into the petcock hole. A pressure washer causes enough turbulence to knock a lot of crud loose. Casting sand sometimes even comes out. But I would suspect something else than to much crud in the block to overheat in just 8 minutes with no load.
