Overheating during cam break in - help

Generally you want it to start quickly and run at the recommended rpm 2000-2500 but you can do that more than once provided the rpm is achieved and its not sitting there idling. the backfire and heat issue could be caused by the timing being retarded , the heat issue could also be an air pocket but if you decide to pull the thermostat that will work , does the engine have a tick? usually when the cam is going it will start to tick and continue to get louder. Did you have the oil sending unit out? other than that it sounds like the leak at the back may be manifold related but look at everything you touched , clean the area and the leak will show itself. This stuff can be a source of frustration at times but with a little time and patience you will get through it. Did you change the valve springs?