Nothing special is needed, most especially "heat resistant" paint.
For restoration quality, there are some professional products made of under chassis components that replicate the oem "satin" finish. Those finishes were not particularly durable but were cheeep which is all that counts in the oem side of the bizz.
I'm working now with a local radiator shop dude who had his f-100 Ford chassis acid dipped, then inspected/welded/repaired in spots, prepped, and then soda-blasted. It's been now coated with a por product which replicates the old oem "chassis black" satin finish. His specialty is resto work on special interest vehicle radiators and knows proper finish for show-quality vehicles/components.
For all my under chassis paint needs (rattlecan style) in my shop is use either wallyworld house brand stuff (dirt cheep) or for the "better" quality I now use only the latest formulations of the rustoleum products.
For engine paint and engine tin, I now use only vansickle implement paint from the local farm supply. It's the highest "solids" (pigment) ratio in the coating bizz than can be spraybombed, takes at least 48 hours+ to flash, but is bulletproof and indistinguishable from powder coat. I also use the rustoleum enamels on engine tin for contrast if desired.