The complete thermostat housings are interchangeable but in some cases you will need matching tops/bottoms because individual parts are not always mix-matchable.
Yes, the diameters of the radiator hoses are not the same. That is why some people use the 235 thermostat housing (that reason, and the fact that an original-style of some 261 thermostats are not very easy to find).
You will have to figure out how to make the lower hose work (if you have an original 261 water pump). I have cut a short piece off a 235 lower hose and put it inside a 261 wider hose at the radiator end (that "bushing" produced a good seal for many years).
In 1954, the 261 color was green, Alpine Green is a very good match.
If no one posts a part number for the full-flow filter cartridge, I'll post a number this afternoon/evening.
