I used an ASD code mk3 derv box, the ratios seem to suit it really well on road.
have driven mk2 and polo with agu box and found 1st and second useless and fifth lacking in top end something like 4k rpm at 80mph which is crap for motorway.
if using a k04 then I would say ASD or CTN code derv box, CTN is slightly shorter ratio.
with a k03 prob stick with standard corrado box
you can then just bolt standard driveshafts up too
FYI mine with a ASD derv box will still spin up to 4th in the wet.
TDI gearbox ratio
ASD Gearbox
• 1st 33/10 = 3.30
• 2nd 35/18 = 1.94
• 3rd 34/26 = 1.31
• 4th 36/33 = 1.09
• 5th 33/46 = 0.71
• FD 60/19 = 3.16
• Driveshaft Flange size: 100mm
1st - 41.803 MPH.
2nd - 70.948 MPH.
3rd - 105.491 MPH.
4th - 150.486 MPH.
5th - 192.293 MPH.
those speeds are out of a gearing calculator which I've confirmed up to 4th