The link Gord posted is clear as mud
You originally provided a link to a thermostat for a 235 so the intent was to warn you.
If the housing on your 261 was changed to a 235 housing, you would need a 235 thermostat. They do get changed - I have a 261 thermostat housing on my 235 and it was a PITA to get the truck cooling properly. There was no thermostat at all when I got the truck - I suppose the previous owner simply gave up. If the housing on your engine is a 261 housing, the thermostat you are purchasing looks right. I have a Stant 13928 180 degrees thermostat. Bypass has to be properly plumbed in or the temperature won't be steady.
