Whoever told you the diesel box gives more torque is a utter SPAK. Only engine mods increase torque
All Ma boxes are interchangable. s1 rallye box is the shortest, iirc on 15s at 60mph in 5th your doing 4000rpm. Not great for fuel econemy but awesome for acceleration. Will loose a fair bit of top speed though...
1400cc 106 xsi gear box's seem to be the best bet for allround performance, slightly better acceleration than a 16v box but not as mental as a rallye box.
Both the xsi and rallye box's are rare as fook, for a good one that still has all its syncros in tact your looking at £150+. You could pick up a cheap fucked one for about £30 and rebuild it but its a pain in the arse, if you pay a garage to rebuild one, depending on what they change you could be looking at a bill for £300+