new guy needs help already

The 4 lugs should be fine even with the V-8.
I know that 86-93 5.0 mustangs have 4 lug hubs. If you do change them, I am almost sure that the front spindles would need to be changes. I do not know about the rear axles though. The swap would not be super cheap because you would need new drums and wheels. If you look on the engine block behind the starter, there should be a # cast into the block. You should be able to read it with a flashlight the first letter stands for the decade the block was cast in C=1960's D=1970's E=1980's. The second digit is the year. Example C8 = 1968 , D4 = 1974.
Hope this info helps.

------------------
67 Fastback - Arctic White Pearl paint
351W 415 hp, Trick Flow Alum. Heads, C-4 Trans, 3.55 gears, Front Disc Brakes, 1-1/8" Fr. 3/4" rear sway bars.