273 Overheating problem

Hi all,

I recently had my 273 rebuilt (bored .030" over, slightly lumpier cam, upgrade from 2bbl to Holley 4bbl, dual exhaust) and am now having overheating problems that are keeping me frustrated. I had the radiator re-cored as well, so I'm hoping that it is not the problem. I can go about 30-45 minutes at highway speed, 2500-3000 RPM with no problems. However, any more time on the road than that and the temp. gauge starts to creep higher than I want it to (200 degrees+, whereas before the rebuild it held 180 with no problems). I can put the heater on high, but that's only a band-aid, it's not solving the problem.

Any ideas out there? Something as simple as changing the coolant, or a different rad. cap - currently a 16lb cap, I believe - ? I'm pulling my hair out, as I really want to get out on the road!