CONCLUSION: a 7 lb cap is marginal if you consider the entire US.
(foreign manf changed US cars 7 lb -> 13 lb... Ford used 13 lb)
The Southwest has hot air temperature, but that's NOT all. The SUN blasts all day and the tarmac reaches extreme temperatures (try barefoot, not certain of the actual temperature, but HOT comes to mind!).
A car driven for hours, parked outside experiences engine heat soak and tarmac heat radiation... and maybe paint color heating.
Technical Details:
180 degree thermostat (carb'ed car)
Antifreeze % was ~10% used mostly as a leak detector in new build.
Cast Iron block and head. (Aluminum head dissipate heat quicker?)
Dark color car (black is seriously hotter than white)