Just to clarify, the MK2 gearboxes will indeed bolt straight up to a MK3 block, but be aware if your old one is a cable change a rod change box will be no good and you'll need to find an 02A cable box from a G60 Golf/Rallye or Corrado (or passat). You could convert to a rod change setup but this would be alot of work and involves swapping the gear lever & pedal box amongst other things.
If your old box is indeed a rod change box, then any MK2 020 will fit, but you need to get the correct friction disc to match the gearbox. If you can find a 16v 2y box it will go straight on without swapping anything, but if you get an 8v box you'll need an 8v clutch friction disc at least to go with it, as the input shaft on the gearbox is smaller.
In summary, buy a 16v 2y, or MK3 GTI box if you want an easy life
