[FFR428]

The 428 has a cyl bore of 4.13 and the 390 a smaller 4.05. Also the stroke of the 428 crank is longer at 3.98 vs the 390's 3.78. Your 390 at .030 is a 4.08 bore. For it to be a 428 it would have to go .080 but not recommended without a sonic check. So your answer is no it's not a 428. The 390 will be fine but you may want a word with the fellow who sold you the car.......G.