[freebeard]

I was thinking about little mini-adaptor plates milled out of billet disks. The Toyota flange has IIRC four studs on a 6.5" circle and the VW has six bolts on 4". I drew it out and there looked to be 1 1/2" between the hole centers. With the nut on the Toyota stud, there would be a counterbore for the VW bolt head underneath.

Or cut and shut, and then send the parts out for cryogenic tempering. There's also a technique where you spin the two halves in opposite directions and then jam them together until they fuse from the heat of friction. You get good weld penetration that way.