GMs are almost all 4 3/4" (5" on "big car" only) and the '75-'79 X-body (Nova, Phoenix, Ventura, Omega, Skylark) front end is the same as the '70 1/2 to '81 F-body. So yes they will fit. Dusters, Darts, et al were the ones that had the early 4", then later 4 1/2" bolt patterns. The rims on the Phoenix may be 14" or 15". I've used the 15x7 rims that came on the later cars on a few circle track cars (and the trailer they sat on - more race spares if needed). I don't know if there are any visible differences between the 68's and the later 70's rims in terms of the style of the rim.
