Unfortunately, I have experienced your exact some problem, times 2. It took a year for me to figure it out. I could not keep it cool. It was a brand new short block I bought when Mopar was selling off their surplus stuff in the early 80's for cheap. Later, a friend had the same problem that we could not fix. He bought the 440 block from wrecking yard out of a motor home. Both of these block were 77 or 78 blocks. Not many knew about this problem and there was no Moparts to ask. Both of these motors had 906 heads on them. I did some research and found that later motor home blocks had extra cooling holes on the surface of the block that mated with two holes in the head that were close to the spark plug. The holes met above the spark plug and help cool it. The heads are regular 452 but are machined different. They have the extra holes to mate the block and use the smaller tapered spark plug the the Chevy's do. We changed the block on my buds and fixed the problem. I found a motor home block that had a freeze crack and bought it for the heads so I finally have a complete MH motor now. That is why I know what the heads look like. My Dad almost bought a 78 MH for the deer lease and when we drove it, it got hot very quickly. I pulled the bonnet and you guessed it, one MH head and the other a freshly painted 906. I told him what the deal was and he said," no wonder it takes two different spark plug for each side." I do not know how to tell a MH block without looking at the surface where the head bolts to. I still have it at the shop, but it has the heads already installed.