All these are good suggestions but here's a couple.
If your car is "almost" overheating as you said, then maybe an overflow tank is needed to recover any lost fluid. Not factory perhaps, but problem solved.
If it's ONLY overheating during idle/slow operations, don't forget the correct pulleys.
Under-driving a WP pulley is fine at 8k RPM, but not at idle. Check both crank and WP pulleys. In this case, size is all that matters
Not to argue the thermostat point, but Ford usually specified a 180-190 deg F T-stat on SBFs. Too low can sometimes lead to unintended consequences.
Correct fan placement should be about 1/2 way into the shroud.
I also agree about the thin, over-bored cylinder walls...most likely made worse when core-shift isn't eliminated in thin-wall castings like the SBF...
Lastly, you didn't say anything about the cam or heads but here goes:
-too much cam duration and the valves aren't closed long enough to transfer heat to the head. Not sure if hard seats make this better or worse.
-valve seats cut too thin (a la racing 3-5 angle valve job) sometimes make the above problem even worse and eventually lead to a burned valve.
I hope its something simple. I love the idea about the IR thermometer. I personally put a 68-GT350 fan/clutch on my 66 Mustang 351W (track-car) and never had overheating issues.
Good luck,
Phil