Man that setup sounds a LOT like mine!!! Solid lifters and the whole ball of wax. I chased overheating issues like you for a long time, never "that bad" but on a hot day always threatening to overheat and running hotter than I liked.
Eventually I too changed water pumps, from the old Ford OEM iron to a new Edelbrock. Whoa, that made a big difference right away. Apparently the OEM style pumps came in two formats, high flow and normal flow. Apparently the newer alloy type pumps are ALL high flow. So, one could assume an OEM Ford pump would work fine IF it was the high flow version, but I have no idea how to tell one from the other.
Anyway, if you have a modern alloy pump already, keep looking for the problem...
Sometime back I blew a hose and lost water and the engine got hot, how hot I don't know, the gauge doesn't read right when your OUT of water!! Anyway, got that fixed, everything seemed to be back to normal for awhile. Then I noticed it was starting to run hot, hotter than normal, before long TO dam hot!! Tried pressure testing the radiator, it held pressure good, didn't leak down at all. Looked for signs of water in the
oil, nothing. Excess water out the tail pipes or white smoke, nope, nothing there. Dam, a real head scratcher. Plugs looked clean all around, compression good. Kept driving it, didn't know what to do, eventually determined than one plug was really really clean and nice compared to the others. Whats up with that? Looked closer, found some traces of water in that cylinder!!!!! A very slight blown head gasket, pushing compression into the water jacket, over pressurizing the radiator system, blowing water out the overflow. BUT, it would only do that when I drove it HARD. Driving normal, it would be OK a long time, get on it hard ONCE, and the temp would climb and it would be hell getting it back down to a good temp. Keep driving it hard and it would eventually blow out enough water it would STAY hot, never would come down.
The freakin' head gasket leak was so small I had to confirm it with a magnifying glass! I couldn't believe my eyes. Anyway, runs great ever since that, go figure...
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