Think simple 1st...
do you have a good radiator..at least a 4 core, clean and flushed. ( I have one .stock)
A flex fan works fine.
Do you have the correct pulleys.? I use a single large pulley w/ a single hemi crank pulley,( about 1/2 space between them)(stock)
Wtare pump..do you have an a/c pump or standard..get an A/C pump if you dont.it has and extra fin i believe.(stock from NAPA)
thermostate...190*.?? NAPA again

I have a mild /hot built 440 AND will run 50 mileS fine in 100 degree temps..at a stop sign still runs finE...never tops 190* (After market guage w. bulb)

NOW when i floor it...i will feel a heat wave like an oven got turned on in the interior. because of the headers..!!!...lol

no clutch fan..(after market flex fan)
the fan is proper distance from the radiator.
And the lower 1/2 of my shroud has been removed.

one thing i havent heard mentioned is .. you sure your timing is right for your set-up?

or (has to do w/ timing).comp ratio..