You mentioned an occasional pinging so more initial timing advance could help that and the overheating. On my old GTX I ran with as much initial advance as I could. When I mixed 104 octane with 93 for a mix of about 96-98, that helped as well in allowing it to run cooler. I suspect the compression ratio was a tad over stock.

Since you're using a direct fan drive with spacer a clutch problem is not the issue. Is this a 2 or 3 core radiator? Has it been painted multiple times? Yeah, once the t-stat opens you should see swirling fluid and bubbles at the top of your core.

But it sounds like a heat gun should identify the hot and cold spots in your radiator or engine. It doesn't take a whole lot of chemicals plating out to get some buildup in those radiator passages.

RR69s