well ................... i did think about making mine 4x4 (it uses mk4/5 front end) - i looked at the standard hubs (the part with the spigot shaft) and it's fairly hefty - my idea was to machine that out to accept a fwd wheel bearing then use a fwd hub of something suitable
so fairly complecated but doable
i never botherd going any further because really i prefer rwd too

i think it would be an interesting project tho
one more thing to consider is the position of the front diff - that might need relocating as the engine is higher in the transit that in the sierra/granada

again not impossable to overcome
