build the car YOU want,as the others have said. unless you are looking to try and make money off this car, which is hard to do anyway, a few well placed upgrades wont hurt anything.
drum brakes all around? upgrade to front discs, they were factory available, and the factory style front discs are inexpensive enough these days t put them on.
want more power? cam, carb, intake, heads, and exhaust mods are all reversible if necessary, assuming you decide to sell the car later on, and you can always save the stock parts so the new owner can install them or not.
suspension upgrades, chassis upgrades, tires and wheels, again all reversible by the new owner.
in the end there are tons of C code mustangs out there, and many of them are either survivor cars, or highly restored cars. so again i say build what you want and dont look back.