The issue with the NP435 from behind the 351M is the input shaft. The 351M/400 needed a longer input shaft to put it in the correct position with the flywheel, where it will be a problem with the 460 as the fly wheel sticks into the bell house a bit further than the 351M/400. I dont remember what the length difference is but it is significant enough to be a consideration for using the NP435 from behind the 351M/400 block on a 460.