thermostat housing problem

Picked up a flowkooler water pump and high flow 180 thermostat for the slant from flowkooler.com. The water pump install went in seamlessly. When I went to install the thermostat, I found it would not fit. First, the base would not fit into the motor. No problem, a few quick swipes from a file and it was in. Second, the thermostat was too thick for my housing. When I tried to put the housing on and tighten it down there was a gap twice as wide as the gasket. Has anyone else had this issue? I don’t think this thermostat is like a crush washer and I don’t want to damage it or strip the threads. The motor was a transplant from a 69 Dart and I’m not sure if the housing is stock or not. I’ll have to look the Mopar part number. Never had an issue with a Stant style thermostat.