I find it hard to believe only one patrol here is suffering from "HEAT BLOCK" after all the problems I had with my 350 chev troopy in a triangle shaped engine bay, Rob Kay from ARB Northern even made a perspex bonnet to check air flow.
To the best of my knowledge I think your problem is your radiator.
all the others with the 6.5 chev have had teething problems but nothing like you are having.
if fitting the vents on the bonnet dont help Id be looking seriously at a new radiator fan set up.
Even the fan can make a huge difference , I fitted a steel 5 blade,, noisey as but cooled down the motor
6 blade plastic was OK and the OE clutch fan worked well also, then you need to have the shroud properly made with the fan
in the right position in with the shroud a little out and youll have problems.
Trust me Ive spent thousands of $$$$ and many different set ups getting it right over many years
it was a work in progress