I'm sharing this as I learn in preparation for my 261 project, so please correct me if I have this wrong.
The 261 also had a water pump with two inlets, one for the bypass thermostat and one for the heater. My 235 has a single inlet for the heater. The 261 water pump can be used with a 235 thermostat without the bypass if the second hole on the water pump is plugged. This is how mine was set up when I found it.
Both water pumps are available and can be seen at the NAPA or Autozone websites if you look up a water pump for a '57 with a 235.
Here is a link showing pictures of a 261 with a bypass thermostat connected to a two inlet water pump with the bypass hose and then after it is converted to a 235 thermostat without the bypass with the second hole on the water pump plugged.
Scroll down to post #19 to see the pictures.
http://67-72chevytrucks.com/vboard/showthread.php?p=6386977Matt
