High Temp's with SL/6 (.060 oversize)

0.060" over size is not the issue. Especially at drive way idle.
Timing - as said before.
Radiator clogged- as said before or too much paint on fins? It takes a lot to over heat a /6/
Lower hose clasping (is no spring internally) When free revving in driveway does the lower hose suck together?
Engine running lean? possibly.
Fan belt slipping?
Exhaust clog? (stretch but has happened)
use an external gauge besides a heat gun. I've seen those fail as well. Especially Harbor Freight ones.
Not much more externally- you may have a bad thermostat. Chinese junk made for a lot of them; it could be stick at idle water flow but opens a bit when driving and more flow from water pump forcing it open.
Keep us posted on results.
Joe