Ed's got the answers! It might be possible to weld in a plate on the older stlye after cutting off the two supporting mounts for the caliper to slide on (this restricts the diameter of the rotor). The smaller mounting tabs for the two caliper mounting bolts might also have to be removed.
I just don't have enough information on the type of spindle (forged vs cast) or enought welding experience to make the call on whether or not it can be done. It also looks like there might be an issue with getting the new holes setup in proper relation to rotor for the calipers.
Randy, if you want to take a closer look, I can send you a driver's side spindle of both styles if you want to split the shipping costs with me. You can overhaul the older style spindle, I just want the new style spindle back unharmed! LOL
If this could be safely done, it could put a lot of old spindles back into action. I worked a little with Rich Thomson at MN12 Performance a while back, but we never got past the design stage. I'm not sure it can be done, but I'm willing to help out if you want to try it. Just drop me an email (address in sig) if you want to persure this further.