I'd be curious to know more about your timing as it can play a major roll in overheating. You mentioned before that you set it at 40 degrees total. Under a load it should be pinging like crazy. That just doesn't sound right...unless you have the vacuum advance hooked up. Disconnect the vacuum advance (be sure to plug the hose for now), get a timing reading at idle, and then run it up to say 3000rpms (or whenever the timing mark stops moving up) and get another reading. THAT reading is the "total" (base timing at idle + the mechanical advance in the distributor) and should be more in the 32-36 range. For a 273, I'd look more for closer to 32-34.
NOW hook the vacuum advance up and get another reading at idle. You probably measured around 8-14 before...and now, with no load (idle), it will probably read closer to 40+. If the vacuum advance is NOT working (or you have only a mechanical distributor...not overly recommended for street driving), your timing is staying low (say 10). This will cause the fuel to burn later and cause more heat to go into the heads and exhaust (think glowing manifolds in the dark)...thus making your engine harder to cool. With vac adv working, the timing goes higher than "total" and ignites the fuel sooner and burns more completely. So, the vacuum advance works only at idle or no-load cruising to create a efficient burn (think: better mpg) and runs cooler at idle.
