fuel delivery issue

Got a gremlin in the fuel system on my Cuda that's got me stumped. The engine is a 408 that produces approx. 450 hp. When it's cool out (75* or less) it runs just fine all the way through the 1/4 and the AFR shows about 12.8 at WOT. As soon as the outside temp hits about 80* the darn thing falls on it's face at about 85-90 mph and I see the AFR meter go full lean.

Here's a list of everything I've replaced

Fuel tank

3/8" pickup (stock location, not a rear sump)

3/8" pre-bent fuel line that has no kinks or sharp bends

Very large high volume 100 micron fuel filter before the fuel pump

Edelbrock #1720 6 valve mechanical fuel pump that says it's good to 600 hp.

#6 braided fuel line to a high volume 25 micron filter screwed into the fuel feed line

#6 Dual feed line to the carb

In addition to replacing all those items I have added a heat sleeve to the lower radiator hose because it's pretty close to the fuel pump and I shot it with a temp gun and the fuel pump was nearly as hot as the radiator hose. Now with the heat sleeve in place I can grab the lower radiator hose with my bare hand when the engine is 190 degrees and it doesn't burn me.

I have also added insulation to the fuel line that's in the engine compartment to keep it cooler.

When it's 80* or less outside the engine runs right on 180 and even when it's 90* degrees out the engine doesn't go above 200 degrees.

Been thinking about adding a rear sump to the tank but I'm not sure it'd solve the problem because this happens even when I have a full tank of fuel.

I have a fuel pressure gauge on the feed line and it shows 6 psi cold at idle and when the engine is warmed up drops to 4 psi at idle. I know that's a little on the low side but since it's a mechanical pump the engine has to be running and when it's running it's using fuel so unless the pump output was deadheaded I don't think it'd show more psi, maybe I'm wrong with that thinking???

I've checked and triple checked the fuel filters for trash/restrictions and never find anything

The only other thing I can think of is the tank is not vented. Could the fuel requirements be such as to require a vent? If so what's the best way to do it?

Any ideas sure would be appreciated. In my 35 yrs. of slinging wrenches I've never ran into a fuel system I couldn't work out, but this one's got me.

Sorry to be so long winded.