Do you notice any steam coming from your exhaust or a sweet smell not hydrocarbon-based? Any bubbles in the radiator coolant tanks while engine is running and warmed up?
I would start by checking your oil then, pull your coil wire and crank the engine while someone stares into the radiator after filling it to see if you get any bubbles. following that, a radiator pressure tester from HF is cheap enough for a tool to keep around to test your cooling system for leaks. Otherwise, could be a myriad things on the 6. @Twisted6 & @Twinshadows , any ideas?The original inline 6