My experience with MH blocks and heads started long ago. A friend had a MH 440 that he bought from salvage yard. He sold the heads and used 906 heads for his Cuda. It ran hot from the start. He could not keep it off 230 degree or higher no matter what. We tried everything. He finally just machined another block and used his heads and all his other components and problem solved. He just threw the block back in the garage and saved it. I personally bought an NOS short block when Mopar closed out a lot of parts for cheap in the mid 80’s. I put 906 heads on it and installed it in a 72 truck for towing. It ran hot as well. 220 was common even with a new radiator. I eventually installed a 400 back in the truck and sold it. I still have that motor as well. Several years later, I learned about the MH blocks with the steam holes that had to use the special 452 heads with the holes to match. I was told water would not circulate good without the match. Now, I live in Texas where 100 plus days are common so it does not take much to get hot down here. I ended up with my buds MH 440 block. It had the extra steams holes in the deck. I sold it a couple of years ago. My block is built with a set of MH heads I finally found with 11.5 pistons. I never installed it in the race car I intended, so it still sitting in the shop. I’m not sure if all motor homes got those extra steam holes but these two do. I can check my casting number but I don’t think the dash number has anything to do with being a MH. They had them for a reason though. Look at a head gasket and you will see the holes I’m talking about.