I don't know about different disc materials. If I read LMGs post correctly, it sounded like a running change from stock that would make an improvement. As a running change, the old compostion flex discs should work fine, it is just the the newer composition might be even better. (if I read the post correctly)
As far as centering - not a worry. the discs have a metal insert that goes all the way through the rubber, and a little shoulder sticks out that nests into your bolt holes. It is self centering. You want to keep the shaft in the same position relative to the trans. for the sake of keeping everything balanced, but don't worry about centering.
I don't think you are looking at a weekend, more like a couple of hours, depending on how fast you can get the car jacked up high enough to slide under and still have some elbow room.

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andy t
'78 300d
'95 volvo 850, wagon
'86 300sdl - engine out, maybe I'll have it rolling by June

whole bunch o' bicycles