I think you have to totally switch out the axles to get the five lugs just like if you were switching to disc brakes. If you are wanting the original "style" styled steel wheels there's a company that makes hubcaps that fit on the 4 lug rims. This may sound stupid at first, but they are extremely cheap compared to the full styled rims, look the same, and you can pop them off for bad weather. I think MustangsUnlimited sells them or it might be NPD. Virginia Classic Mustang has a set of 4 for $189.95 center caps included. Their phone # is 540-896-2695.
These might be an option for you. Hope it helps a bit.
Good luck!
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
Both are slow but easy on the eyes!