I found a wacky little red o-ring from a Mini parts kit that shims the whole thing up fine. Fits, seals, the works.
This whole thing has me wondering if it ever really mattered- the same valve fits inside an actual WGP RF the exact same way and from what I remember the gun shot fine.
I haven't really touched Autocockers since 2010 but I didn't remember the valve and body holes not lining up / the valve retaining screw tilting the valve inside the tube THAT much. But hey- if it seals and shoots.
Ordered a different valve just in case my solution shoots itself broken.