Many people have done it with many different products. I used POR-15 with a black top coat. Some have first used rust remover or reformer followed by spray paint.
I used SEM Ez coat to paint my axle a couple of years ago, it's held up great. The color matches Fords factory frame and suspension part color. You can get this at auto body supply stores in comes in spray cans and is perfect for touch ups on your frame and axle.
Just use normal rust oleum.
I would suggest semi gloss, or gloss.
The mat or the flat, is very porous if you will and will collect and show dirt more readily
I've used basic Rustoleum multiple times on the frames and axles of my Toyotas and Jeep with great results. Only used hi temp on the exhaust which also worked well.
I used Corroseal to convert the rust and it provides a protective base layer. For paint I like both Spraymax 2K hot rod black and Eastwood 2k chassis black. The Eastwood is nicer but expensive and there’s not as much product in the can. The Rust-Oleum rust reformer is really good too.
Painting pumpkins is for sissies. Real men carve pumpkins:
Kidding aside, I love this idea and want to do it. I will propose that a rear diff cover is probably a good first step, as you'd hate to paint and then have to decide if you want the new cover to match or not. The Ford Performance rear diff cover is just a Spicer cover with a $100 Ford Performance name badge riveted on, so get the Spicer. It may or may not add structural integrity, but it does make draining and refilling the fluid 10x easier. I haven't personally used POR15, but folks who have seem to love it.