I've recently been through this myself and learned a lot. The 261 is very different than the 235 when it comes to the thermostat and housing. The 261 has a dual acting thermostat that allows some circulation through the engine during warm up that avoids hot spots until the main portion of the T stat opens. A useful feature I decided to retain although many go with the simpler 235 set up. The 261 will be tapped for a hose barb in the upper and lower potion of the T-stat housing and an original 261 water pump will have two outlets to feed the system although as I have learned a single outlet pump will work if you tee the line. If you decide to go with the original system I have the part # and make of the dual acting thermostat that I was able to get at NAPA.
