The disc brake conversion will fit to your G2 Syncro whichever (68mm or 72mm bearings) rear arms you have, so you do not need to swap them over for Rallye ones.
To replace rear drums with 239×10 discs,
1) 2 of brake discs p/n 811 615 301, this is actually the same disc as fitted to the golf 2 1050cc front wheels,
or you can fit the 239×12 mm front discs as fitted to the Golf 2 1300cc to 1800cc models (55 to 90 bhp) to the rear of your Syncro, P/N 321 615 301
You will need to space the caliper carriers out from the swing arm (as the brake disc less offset) I find that the Golf 2 Diesel exhaust manifold washers are ideal for this P/N – N 900 955 01 (4 of)
2) 2 of calipers p/n 535 615 423x left and 424x right, these are the same as fitted to the rear on a gti 16v and most other vw rear disc applications of that era,
or you can use Golf 4 aluminium calipers also fit and do give better performance and have the bonus of a none seizing handbrake mechanism,
Golf 4 Calipers p/ns 1H0 615 423B left & 424B right, you will need to modify the brake pipe caliper end as the G4 uses a different fitting, but its not difficult all you need a male to male adapter piece.
3) 2 of Caliper carriers are p/ns 191 615 425a left and 426 right, and have been fitted to all car type syncro rear discs from 1985 to 1998 thats the Golf, Passat and Jetta,
4) 2 of Golf Rallye cables, p/ns 191 609 721H left & 721K right,
This may help http://www.vwsyncro.co.uk/data/forums/topic.asp?TOPIC_ID=119
