If it was my engine I'd take the 261 t-stat housing off and put the correct 235 housing on. Or connect the by-pass back to the water pump like it was on a 261...
The 261 housing has a by-pass outlet the was connected to the top inlet port on the water pump with a short 90* hose. This allows coolant to circulate through the block while the engine was warming up. After it's warmed up, the t-stat opens to the radiator and closes the by-pass outlet putting full flow to the radiator.
Do a Stovebolt search, there was a good discussion about the 261 t-stats 3-4 years ago, it might help...maybe Tim remembers...
Mike B
