Don't have to change cam if you don't want to just the lifters. Ok to put new lifters on a flat tappet cam but not ok to run used lifters on a new cam.
Lifters just gotta have the pisser hole to feed the pushrods with oil. And on the 3 magnum heads to LA Block jobs I've been in on 7.650 seems to be the magic number on pushrods. The MP conversion pushrods are 7.625 and each time I've tried them they've been too short, wind up with lifter clatter like you'd hear on a /6.
There is a ford 351W application that uses that length which were the pushrods I used, so being an off the shelf application instead of something custom also means the price of these pushrods are reasonable.