There is a place called: www.Stocktonwheels.com they have magnums rims up to 17" I called them and asked for a set of 17X8 complete, centers and valve stems $1225.00 without shipping. I don't know how much 16" will be.
Center caps, either the Mercury head '67, the 69-70 blue walking cat, or any of the Ford styles are $30ea.
So you can see if you get a kit of all four, you get the caps and lugs for free, and even get a price break on the four wheels!!
They also have wheel and tire packages for the 14x6, 14x7 or 15x7 with BFG white letter tires in 185/70 up to 225/60 series, center caps and lugnuts (and of course valve stems), all mounted and balanced for $899
This is an example of what sort of package deals you can get if you look from the Mustang vendors.
Yes, actually I did! The whole reason I picked it up is that I was looking for a replacement hood lip molding for the wife's 70 Mach, and the NOS molding I saw were waaaaaaay overpriced and had the same dents (or worse) than the one on the car now! CJ Pony had repops that looked real good at 1/10th the price of the NOS junk out there.
Still, if you're looking at 14 or 15" wheels, they are not so bad these days, pricewise. If you pick up a copy of Mustangs and Fords or Muscle Mustangs and Fast Fords you'll see all sorts of vendors out there with wheels and other parts that fit our Cats!