[T77911S]

i seem to remember someone putting the engine Tstat back in wrong and causing heating problems so i was trying to search for it. not a fan of searching this forum. i did not find it.
but some other ideas.
wires to the temp gauge and pressure gauge crossed.
this seems quite obvious but someone did this. no hose from the engine shroud down to the heater boxes. air blowing into engine bay.
of course as others have implied, you are not really overheating. check temps with thermo.
waaaay too lean. timing too advanced.