If you just plug the hole without a bypass, then your t-stat won't really see enough hot water flowing past in order to open properly. They have a tendency to run up to about 230 or 235 deg. before they open. Then the motor gets a big slug of cold water and then the stat slams back shut. I've got a 74 3/4 ton just like your 78, and I had to do the same valve thing because the door rubbers are deteriorated. That took away the bypass, so I had to make sure it had another route to recirculate the coolant.
What you described with a hose outlet on top of the pump and another in the t-stat housing is exactly like the stockers came from the factory. The kinked hose to connect them is easily found at the parts houses.
------------------
David
newbie #4153
