My 1.8t tip has been slipping for a while now and recently starting sounding like rod knock that I'm hoping is my flex plate.

Anyway,
Found a 2.8 trans from a full part out.
Most posts I'm seeing say it's a direct swap, some mention the 2.8 side Trany mount and final drive, but I believe that's only for auto to manual swap?
And how about the flex plate? interchangeable and maybe a place to buy new?
Anything that needs/should or is convenient to replace at the same time?

 

Lots of little internal differences, mostly in the number of clutches and bands in the various gears. Torque converters are different (by part number) don't know how much of a physical difference there is. What year and tranny code is in the 1.8, and what year and code is the 2.8L tranny? You may find some with different final gear ratios which will require changing the rear diff as well (I'm assuming you have a Quattro?).

I've got the same issue with a 2001 A4 with an FAL coded auto trans. Seems everyone that has an FAL tranny for sale thinks they are made out of gold, and worth more than the whole car. So if this works for you let me know, the 2.8L ones grow on trees and people will practically pay you to haul them away.