The top part of a GMC inline thermostat housing has a pipethread cast into the part on the left side of the housing. This is the perfect place to run a hose or tubing from to the front opening on the manifold. If you use hosing, try to get into someone's catalogue and find shaped hosing so the bend is made into the hose. It will look much better and look pro. Also, only a minimum amount of hose will be needed. On the rear opening of the manifold, the outlet side, instead of going all the way to the heater, install a "T" at the temperature guage probe and, once again use the pre-shaped hose. The hose will look good, but for safety and a less likely chance of a leak developing, the tubing is better, stainless or aluminum.
