you must have 75 degree t-stat. 180 F is about 82 C, which seems about right. the temp at the t-stat being 75 degrees would probably heat up to 82 degrees after flowing through the engine block.
btw i am running a 75 deg stat, and i do recommend.
And yes, everyone else's M42s on here seem to have overheating problems. Consider yourself lucky.
I may do something similar to what you did, and if i do, i will make a write-up with lots of pretty pictures.