The key words are "used to be behind an ax-15."
If the 30rh is 23 spline (probably, I think all 91+ trannies are 23 spline), then the d300 (if already flipped with a kit) should bolt right to it, but you'll still need new driveshafts and shifter linkage.
If you have some fabrication skills, there are writeups on how to make your own d300 flip kit, but if you're lucky the PO already installed one!