See if you can figure out which pan you have. Just cause its a "69" doesn't mean its not a different year pan. Check the VIN under the backseat, should be able to tell you what the chasis year is. If it is an actual 69, then you have the mounts for the IRS transmission which 1) are cheap and easy to find all day long, and 2) don't have huge camber issues like the earlier swing axles do. If it has those mounts (near where the frame horns come out from the torsion housing), you could convert it back for minimal costs and probably trade the gear reduction box set-up for it or break even. I wouldn't bother trying to make it work.
For what its worth, lots of people have taken them out of buses because it greatly limits your ability to keep up with traffic. I think top speed is right at 55 or 60 mph and thats pushing it. But if you are going to keep it a baja, then it would be nice to have the lower gear; I've been the baja jumping up hills and rock faces and you break a lot. I would have loved to have a lower gear. Post up some pictures of your bug. Is the rat rod going to be for the 14 year old or do you have two projects?