Both of my experiences with the MH blocks were similar. One was a bud of mine. He bought one from the local yard and installed 906 heads on it. Could not keep it off 230 degrees. Changed the block and left everything else the same and ran cool. Mine was an NOS short block I bought when MoPar was giving stuff away practically. It was a cast crank std rod motor. It was in a tow vehicle with 906 heads. It ran hot and would vapor lock all the time. Finally gave up on it and the 400 back in the truck and sold it. They were both 76 or 77 castings. I final bought a 78 broke motor home long block with correct heads and they are on my motor now. It runs fine.