Hi,
A 66’ Pony’s (4) lug pattern is 4-1/2" In., Foxes run a 4-1/4" In.(4) lug pattern (79’-93’). I believe you were also asking if the backspacing & tire would work (IF the pattern). You’ll be hard pressed to shoehorn a 275mm tire within a 66’s stock rear/well on the most ideally backspaced Rim.
The 66’s are, unfortunately, not overly accommodating for wider tires as the 68’s-69’s.
Rim backspacing to 5”+in. (tight to your springs) some well tweaking & lip rolling will be needed to run a 255. There’s an adapter kit that would allow you to run later (SN95) model (5) lug rims on your existing setup, here..
Note: Hate to leave you with nothing.. there are quite a few vehicle possibilities that share the (4) lug pattern your 66’ requires should you be seeking tires/rims to be able to move it, these would be common at a local auto recycler. If you need the list, just ask...!
-John
One of the reasons everyone with a 4 lug vintage mustang upgrades to 5 lug..that, and better brake options. There may be other options for you though...I would check bolt patterns for old 240zs and RWD Celicas....I would think you could find a Minilite wheel in 4x4.5" pretty easily