smart thing to do here is obtain an oil pressure gauge and confirm that
you have an oil pressure problem, not an oil light/sender/wiring
problem.

The 'default' position for the cooling flaps is wide open, although you 
cant trust this on your bus as if they havent been used in a while
they could be stcuk anywhere

they certainly can make your engine overheat if they are closed - in any
climate - btw i think a lot of people overestimate the 'load' put on air
cooled engines by the hot climes - the impt parameter after all is the
difference in the air vs engine temperature, so 50 degrees differnece in the
air temperature doesnt have as big an effect as some people have 
suggested.

I would certainly check the sending unit , and the rest of the
circuit before i worry about the innards.

i have run bugs ini buffalo,ny without the cooling flaps so this
is certainly an option for you in sunny TX!!

7.5 BTDC is a good starting point for timing - absence  of pinging
says this okay.

did you replace the oil pump - or at least grind the gears flush with 
the end plate?  You could be losing pressure here.

hope this helps  a bit