Reply to "How hot is HOT?"

The 'normal' water temp for a nearly stock 351-C in a Pantera at cruise speeds is 200-210F; changing to a lower rated thermostat will do nothing, in my experience. The '71-'72 gauge is 0-230F while a '73-up gauge is 0-260F and is a 'stock-upgrade' that often reads closer to true temps. Most of the problem lies with the sender; the oil pressure sender is similarly 'off' so your pressure will often show far lower than is true. A poor ground on the gauge(s) or a particularly inaccurate sender(s) will give the above problems, as will a worn-out thermostat. As Kenn said, I'm one of those who gets about 1 yr out of any given thermostat.

Another cause of climbing water temps is a mis-timed ignition. If your mechanic wasn't specifically looking, the 40-yr-old harmonic balancer on the crank may have slipped it's position so the timing mark shifted. If he then adjusted it to the mark, the engine can be as much as 10 degrees retarded, where it will run hot.
Much the same thing happens with over-aged timing chains- the chain wears due to the easy high rpms possible with a 351-C along with the very heavy valves. The result is an engine that has it's whole valve train retarded (relative to the crankshaft) on acceleration, and advanced during deceleration- exactly opposite of what you want. This also induces engine heating. An average 351-C gets roughly 20,000 miles out of even a heavy-duty timing chain, and less from a stock chain.

If you do everything suggested above and the car still overheats (and by that I mean it spits coolant out the overflow, since your gauge is not trustworthy), the ONLY fix is a new, bigger radiator along with much larger sucker fans mounted in a shroud, replacing the stock toy pusher fans. I have tried absolutely all possible upgrades to control water temps in our '72 L, and replacing the rad' plus fans were the only ones that actually lowered cruise water temperatures. There are good reasons why most Panteras are not stock....