For those with Magnums - What Mounts did you use?

Let's say you're Chrysler, and you make 360 blocks with one mount, and then 318 blocks with a different mount, and you sell a few million of them.

If you spontaneously change the mounts that the 360 uses then you have a problem when one of your existing 360 customers orders a replacement block. The new block doesn't fit the existing mount.