Sorry to hear you’re still fighting that, always welcome for the response! She certainly likes to run warm under a load, I see, lol! It’ll be nailed right quick- no worries..I’d had a thought regarding your fuel pressure, are you certain it’s maintaining between 5 and 8psi at all speeds, maintaining that pressure under all load conditions, because if it drops off at mid-higher rpms (Even Low Float levels, Motor uses more fuel when under a load) running the risk of going lean. ‘Near stock’ Carbureted engines like yours use roughly 14.1:1 to 13.4:1 air/fuel ratio at idle and slightly richer at high rpm’s. Lean will ramp engine heat up very quickly...
I’d prev.asked what #Jets you’re running in your Primaries (?) Pull your Plugs out and read them after a quick cruise. You should be Tan to a little bit of Carbon, White or Ash Color indicates lean, (good charts with pics of Plugs online under various conditions). It won’t hurt you to bump up a couple Jet sizes if need be- for Testing. We can get further into Carb Tuning, should you suspect, or you’re Plugs do read lean.
The other is your dynamic Timing advance rate possibly coming on too steep, so- too much Timing thrown at it too quickly (will still run very well) & also cause sudden uncontrollable Combustion Chamber Temp. spikes similar to what you’re experiencing..What Distributor setup are you running, it have a Centrifugal advance with Springs, weights & Shims to tailor your Timing curve, and also using an adjustable diaphragm Vacuum advance ?
Test it out, Hook up a Timing light- mark the balancer: @ TDC, then idle timing @12 degrees BTDC, then maximum advance @34-36 degrees BTDC. At operating temp and snap the throttle open and see where you are..How fast Timing is coming on in the RPM range is my concern, but too much TOTAL Timing will do the same, verify your Idle @ 12 Degrees BTDC (Or what your target is) & High RPM Max. of 34-36 Degrees BTDC (about right for most near Stock 1960’s Ford V-8’s). If it exceeds that, we should talk Cam Spec’s..
Try holding your throttle at 2,500-3,000RPM’s under no load for 2-3 Minutes & see what your Temperature does in that scenario..btw- Never intending to insult with questions- just trying to help & not overlook menial possibilities -
. Is there any chance your Fans are not rotating in the correct direction? I’d also verify that you’re pulling a healthy 15-22 Hg’s of Engine Vacuum at Idle..Cam’s not that radical-correct? Post what you find if you’d like, I’ll see it- take it from there...
I like to keep a set of Mechanical Gauges Plate mounted Underhood for Tuning, nothing special- Temp, Oil Pressure, Vacuum- really does come in handy in situ’s like this. Good luck! Cheers!!