904 to 518 swap....??

Yes as TopHat said it's not something you should attempt without help from someone who knows how to rebuild the cross member top half after you cut most of it out. This is the torsion bar mounting point and an important structural point of the frame. You also have to build a lower cross member to mount the tranny. I have looked into this as I have a 518. If you decide to do it PACT 700raptor.com has a kit to make the overdrive work off of the hydraulics and electrical so it shifts pretty much automatically in and out of od. I have some pics from a guy who is doing the swap, pm me if you would like them.
Bruce