I'd agree. Having said though, if the high temps have only happened once he chucked a condenser for the air con in the way of the rad and ic. The problem is fairly obvious. You need better airflow to the front.

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Ongoing Project: 1972 Datsun 510 S13 SR20Det. http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18

Couldn't agree more.
But do some testing like the previous example! (not necessarily put a different thermostat), more make up some ducting and put a thermistor before and after and measure the temperature difference. Exactly what you have suggested.

Trial and error is the way, in a methodical manner... because we don't have the billions and the staff of the big automotive players...

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
viewtopic.php?t=6579
#SR20Datsun @SR20Datsun

well my solution seemed to work. swapped to 2 10inch thermos, a high flow thermostat and high boiling point coolant. cruising with aircon on im seeing temps drop 20c then what it was before. running around the 100c mark that is on a hot day. with aircon off temps around 90c. the highest im seeing now is about 110c with aircon full on a hot day and giving it at 10psi so im much happier with that now

A good outcome.

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
viewtopic.php?t=6579
#SR20Datsun @SR20Datsun

Good work, thanks for updating the thread.