The 4.25 can be modified to a 4.4. I have not done it, but I would imagine it is easier with a stock that does not have a pillar installed, like a boyd's or Bell carlson. If you know what you are doing, and have one with a pilar, just fixture it real good steady under a drill press and may have to make the hole a little bigger to remove the whole pilar.

When I was looking into it, it seems like it is just easier to get a 4.25 action. All of the above info you stated is correct, for a short action, long actions have the same spacing from stagger to centerfeed.