Im kinda having the same problem,but my radiator is the old honeycomb type. Its in my recently acquired 52 suburban with newer model 235. So, im gonna recore the radiator first. But ive been following the threads on pulley sizes and I'm now wondering the same thing,,,,is my pump spinning too fast?
Even starting out from cold in the driveway,and going only a mile or so up the road,at 25-30 m.p.h.,Ive already reached thermostat temp.(180)
Suggestions before i recore?
Thanx.........

Marty
'54 Panel and '54 5 Window Pick up, in the Gallery
'52 Suburban