Are you sure you're thermostat is operating properly?
A super high performance engine generating a tremendous amount of heat would need higher flow to draw away excessive heat.
Don't confuse temperature and radiator cooling system internal pressure.
Increasing the internal cooling system pressure results in increasing the temp at which coolant boils.
What thermostat temp are you using. Thermostat is correct fit installed with correct gasket or "O" ring style. I found once wrong gasket installation causing overtemp. The pressure contained in the cooling system is limited to what the cap is rated. Expansion recovery tank retains some then returns to system when pressure reduces. You said it has high pressure? How do you know? Do you have a pressure indicator or pressure gauge on cooling system? if the cap operates as designed the max pressure is never greater than the caps rating. What is your systems rating, GM truck, radiator manufacturer? Have you inspected the cap? Proper fit? Cap seals whole no cracks? Are you sure the aftermarket radiator and cap are compatible? Tested cap? Holds and releases pressure at rating?
And your sure there are no obstructions in the radiator, proper cross flow.
Is your radiator the same size, capacity as a big block, tow package , K2500, suburban would have? As a equivalent example.
The 1/4 " difference is not a factor in your build. The perceived reduction in volume is negligible, your flow is no where near maximum velocity for the diameter.
What clearance did you use for your extra fluid coolers in front radiators/ ac condenser? From what you've described I've tried to cover some possibilities.
Good luck !