Looking great so far. I love the preservation approach. Those wheels/caps are going to look awesome with some white line tires! For some reason to me, the 67/68 GMC in this color combination is one of the best looking combos from the 67-72 era. I forget who's truck this is, but it's one of my favorites.
Here's a photo I found of an all original '68 GMC wheel/cap. The colors look the same to me. I agree with above, your cap has probably faded compared to the new Argent on the wheels.