I have built a few 454 Mark IV engines (back in the day). One of the things I noticed about every one of those 454 engines is that they had a lot of casting flash blocking the coolant passages in the block and head. When I would build one of those 454 engines, I would de-burr all the coolant passages between the head and block, and clean out the blocks coolant passages as good as possible, which seemed to help a lot with the cooling issues 454 engines are known to have. All of my old Mark IV 454 carbureted engines ran pretty steady at 180 degrees under all conditions. I am not saying this is the problem with your engine though.