The summer heat can tax even the cleanest of cooling systems. A lot of things can contribute to running a little hot; gunk in the block, radiators partly clogged, etc. Start by knocking out the freeze plugs and cleaning out the block; a very nasty job but absolutely needed. Have the radiator taken apart and cleaned (and it might need a core) and possibly a larger core or radiator may be needed. On the 42 you may need to pull the water distribution "tube" and see if there is anything left of it. A new thermostat is in order regardless of bolt troubles and also check your timing, total advance, and general state of tune. A ton of discussions on cooling system troubles exist in the old newsgroup posts, so if you want to go through them and get some more insight, that would be a good place to start. Our cars came from a different time and don't like to idle in traffic on hot days, and even the smallest restrictions in the system or other problems can cause them to run hot. Also on the 42 I would pull the pump and check that the gasket is on correctly; for some reason I have seen a few that weren't on right even though it is a simple job. Good luck, Studebaker George