I brought up the water pump inlets in relation to connecting the thermostat housing as an fyi that I wanted to get posted just in case anyone else tries to figure out where the bypass connects to. By getting it posted, it becomes a permanent part of any internet search for anyone curious about the same thing in the future. I thought that the link that I posted to the pictures would be helpful. I didn't mean to wander off topic.

Hooking up a 261 with an original thermostat becomes more complicated than just finding the correct thermostat and housing...hose sizes different, different water pump, etc. I am learning! I might end up just installing a 235 housing for reasons you mention.

Two other small things that might be worth memorializing for future searches with this post. One, the 235 top radiator housing has a threaded hole that holds the clip that secures the fuel line and vacuum line as it crosses in front of the valve cover. I don't think that the 261 housing has this based on pictures that I have seen. Two, the 261 (at least in 1957...my truck year and my engine year) not only came with the larger hoses that Tim mentions above, the different water pump and the different thermostat and housing but it also came with a larger radiator, a fan shroud and a 20" five blade fan. The radiator is about two inches wider on each side. It fits the same bolt pattern on the core support, but the sides are at 90 degree angles to the bolts instead of being smaller than the length between the bolts with angled brackets to the core support.

The entire 261 cooling set up seems like it was engineered for extra cooling. Here is a picture of the radiator, shroud and fan as I found them on the '57 6400 that I pulled the 261 from. The radiator was shot, but I did scavenge the brackets from it that hold the shroud. The brackets, shroud and fan were easy bolt-on's for my 235.

http://s844.photobucket.com/user/ma...1170_zpsla45xxqs.jpg.html?sort=3&o=6

All that being said, I am considering just using a 235 thermostat housing since I can't recreate entirely how the GM engineers designed the cooling for a 261without the radiator. The other parts of accessible.

I have learned quite a bit about the 261's cooling system along the way and appreciate everyone's help and advice.

Thanks,

Matt