The 5-lug truck Rally wheels are (stock) 15x8. I have a set of "van" Rallys on my '72 (what's "left" of my '72

). They are 15x7. Both of those have about, if not exactly, the same size slots in them. They both have the 5-on-5 'truck' lug pattern.
I don't know about the 6-lug Rallys, other than they came on 1/2 ton 4x4s. They will bolt directly up to the older 6-lug pattern hubs ('60-what to '70). I'd like to have a set for my '66, but haven't really been looking.
I used to have a set of 'car' Rallys on my old '69 Chevelle. They were 14x6, but they were the 5 'slot' car type Rallys.
Gotta love those Chevy Rally wheels.