I know a little (Well enough to get my self into trouble), and hope to know a lot soon (fitting a T4 to my beetle), but I have read that you can remove the T1 throw out bearing pad on the pressure plate and fit it to the T4 clutch, or fit a 1971 and on T1, T2, T3, throw out bearing support tube. The other part you have to change is to fit the T1 pilot bearing to the fly wheel which is just a case of machining a bigger hole to fit the T1 style needle bearing and press it in, it should still stand out a little I am told.
Also try the Type4rum.com for loads of info... loads of real T4 tech heads there.
http://shoptalkforums.com/viewforum.php?f=1
Hope this helps sorry I do not know any T3 stuff.