The backing plates were usually dipped in semi-gloss black paint, the appearance will vary, but generally will not match exactly to the rear end housing. I would not use the same black for both.
On the rear housing, a semi-gloss black is fine.
Powder coating will be more durable, but could fill in stretch marks and other manufacturing details on the housing and backing plates.
The best, most correct appearance, will be using epoxy primer as a base, followed by a catalyzed single stage urethane. The trick is to not get a high build-up of paint. If there's any pitting in the metal, care will need to be taken to not fill in things like FoMoCo stampings in the backing plates. If I have to fill in pits, what I normally do is apply epoxy primer to bare metal, then tape over the stampings before spraying build primer. Pull the tape off and then sand to smooth out any pits.