In an older car I would have retained the short water pump, that the Deluxe Kit ships with (assumes you bought the Deluxe Kit that is complete from carb to the over sized truck oil pan). It yields more room between the fan and the radiator.
I also feel the flex fan will create issues besides the fitment issue with the alternator pulley. Flex fans do not save energy over a fixed bladed fan despite claims by the retailers. It takes energy to straighten out the flexible blades which deform as well as bend. When they deform they no longer form an air foil to move air and the bending creates shear forces that will cut through aluminum rivets.
GM tried out Hayes flex fans back in the nineties and then recalled every vehicle that had one installed and replaced them with a non-thermostatic fan clutch and a new Hayes Fixed pitch six bladed fan. I pulled out of the dumpster ten at a time and resold them later on at flea markets, but that is another story. GM replaced these fans because they shed blades at high RPM, and newer EFI powered engines where picking up in horsepower as emissions fell creating a renewal of interest in hot rodding.
Here is a picture of a flex fan that had been installed on a 406 SBC in a 1978 Nova belonging to a former Chevrolet line tech who used to replace those flex fans and might have kept one for himself.
All fans have a set RPM limit. Flex-o-Lite determined through testing that most fans were at risk of failing above 5,600 RPM. Even if they didn't throw blades like a deranged Ninja they would go into cavitation which prevents movement of air and creates a lot of noise (big semi's cavitate their fans at the top of every gear, which is why the roar of the fan exceeds the noise of the engine at a stop light drag race. If you use a fan clutch you limit the peak RPM of the fan blade at about 4800 RPM. If you run a thermostatically controlled fan clutch you save 80% of the energy to drive the fan as it free wheels at cruise and when cold.
As to your radiator the original 425 horse 409 A/C equipped radiator should keep your 502 cool with a fan clutch and a seven blade BBC fan. Though an aluminum radiator can be custom built to your heat load and car restrictions as to size requirements.