I'm running a stock TPI and TPI water neck. I can't get it to build heat when ambient temps aren't hot without working the motor hard. I've installed two different thermostats with the same result. I'm using a standard SBC thermostat housing gasket, but I noticed that there is an option for the rubber seal that goes around the outside of the thermostat. What is the proper gasket/seal?

This problem isn't present when the temps are 60-90, but at 40 degrees it can idle all day long and never excedd 150 degrees, which leads to me to think thermostat, and becuase it has occurred with two different thermostats I'm wondering if the issue is the seal (or lack there of) that is causing coolant to leak past the side of the thermostat.

I'm looking for the correct factory setup for my factory setup.
As always, advice is appreciated.

And, FWIW, it will heat up with some spirited driving for 1-15 minutes and doesn't overheat - we're making about 450-550 to the wheels with 14 psi of boost.

Again - thanks.