The 390 is a great motor. But on a budget, IMO you will be way better off building a 5.0 for that mustang. Get a good roller short block, E7 heads (all dime a dozen at pull-n-save) and you are off to the races.

460 is a different engine family (no interchange) A 428 crank is a good way to go, but not common and expen$ive.

If you are not sure you have a 390, don't be surpised to find you have a 352 or a 360. There is no good way to tell without pulling the heads.