If I understand your description, you can keep your carrier bearing and rear driveshaft, then you will have to have a driveshaft made or shortened for the middle. I think having one made shouldn't be too much more expensive than finding an intermediate shaft at a pick and pull and having it shortened with new Ujoints. Certainly save you some time. Should be fairly easy to find the rear bearing retainer for the SM420.
Is something wrong with your TT? All I can think your would gain would be easier servicibility (sp?) should something go wrong in the future and a bunch of jugs of used 90 weight.
