Lou, slap me if I need it, but I assumed I could change out the pump without removing the housing. No?
Not sure what happened to Lou, but for sure you can change the pump without removing the housing, or upper hose, or thermostat. That's not to say that changing the thermostat and hoses is a bad idea, while you have the cooling system drained, but it's not a requirement.
Many will disagree with me, but I've never found a cooling advantage to using the bellows thermostat either. I've tried it, every time I've had cooling troubles, and never been able to tell the difference. Once the cooling problem is fixed (for me, usually either a leaking head gasket or a bad radiator, although this last time it was leaking valves), going back to a modern was pellet thermostat also made no difference. I've even gone as far as totally blocking the bypass (the bellows thermostat only partially blocks it) and drilling a hole through the thermostat (so there is always some water flow but it goes through the radiator instead of the bypass). It slows down warm-up as expected, but still doesn't seem to make any difference to overheating.
Of course, my cooling system isn't totally stock either, so YMMV. The crank hole was one of the first victims, I happily left it out after my radiator guy said it reduced cooling capacity by about 10%. I also removed the mechanical fan and rely on an electrical fan; which never runs except when I'm stopped at a light or just pulling away from one. It's set to come on when the water out of the radiator reaches 190F, which almost exactly matches what I see on the temp gauge (which shows the temperature going into the radiator). So it seems clear that the issue is only lack of airflow, not any lack of coolant flow. And it takes less than a minute for the temperature to drop, which indicates that even at idle, I'm getting pretty good coolant movement through the radiator.
Hottest weather I've driven through was 115F, rush hour in Palm Springs. The driver overheated before the engine did! (We rented an air conditioned room and waited until the sun went down to continue our drive.) I've also done some desert runs at close to 100F and 100 mph. Engine stayed cool during the run, although the temp gauge did go up temporarily after I slowed down.
PS, even the factory went to a non-bellows thermostat later on, and listed the wax pellet thermostat as an approved replacement for earlier cars.