Rallyes
So I can put the E-Body styled center caps on no matter what bolt pattern my car is?
Yes, they make the early style caps in both bolt patterns, 4 inch and 4 1/2, you're good either way. The best way to figure out what you need is to tell us if you have front discs or not.
The '73 Duster I had was a 318 3-speed Duster Twister, it had the 4 1/2 ralley wheels from the factory, before I got it someone had added some OE early style caps.
From what I've experienced, post '72 ('73 +) slant six drum brake cars had a 4 inch bolt circle. All V8 cars I've seen, '73 + have had the 4 1/2 bolt circle with front disc brakes. You could get disc brakes on a slant 6 car, pretty sure all '73 + disc setups are 5 X 4 1/2.
BTW, are you on the SAE (or SA for the young'uns) forums?