Hi There,
The best looking finish and paint to use would be satin black. A 2Pak satin black enamel is the most durable paint to use. That's what i used on my front bumper bar resto. You can buy 2 pak satin paint pre mixed to satin finish from a good auto paint store. Mat black is acceptable, but satin black looks the best and most classy in my opinion. Glossy black looks cheap and rinky dinky, again in my opinion, but it's what rocks your boat that matters i guess.
If 2 Pak is too hard to spray with your home setup, then move into a satin black acrylic lacquer paint. Not as durable but will work fairly well also. Spraying with a rattle can will get you out of trouble, but a better finish is gotten by shooting from a spray gun, and mixing up your own lacquer with quality thinners. You might want to use a primer first though, to achieve a top looking final finish. Remember, lacquer primer for lacquer top coats, and 2 pak primer for 2 pak topcoats, with the exception being that you can spray lacquer top coats over 2 pak primers if you want. No need to use flex aids either, as the paint alone will withstand the amount of flex back there.
Here is a link to an article i did on restoring my own '73 front bumper bar back in 2014.
https://www.7173mustangs.com/thread-austin-vert-s-front-bumper-makeover
Hope that helps,
Greg.