427 has a 4.234" bore by 3.78" stroke. 428 has a 4.13" bore by a 3.98" stroke. In essence you can't make a 427 out of a 428. You can't bore a 428 to 4.234" There are many other differences, cylinder head options, crossbolt mains, side oiler oiling for some 427's, most 427's were not drilled for hydraulic lifters and there are more differences but that is the meat of it.