http://chevy.oldcarmanualproject.com/shop/1959trucksup/59ctsms0813.html

Describes the 261 cooling system... It sounds like getting the proper t-stat is gonna be a PITA. Its much more than a simple hole in the t-stat.... but I would think that it would work.. as long as you have water moving in the block to remove/prevent air pockets/steam from forming.