the cage will be tied into the rear subframe bracing, yes. we're using square tubing (don't recall dimensions at the moment), and we'll be making brackets for the rear diffs' subframe to bolt up to. decisions have yet to be made, but i'm thinking that i want to remove the rear seat upright and deck, and place the fuel cell there. this way, it can stay within the cage, and still low to the ground.
haldex controller would be nice, yes. there's a couple other options out of europe that i've seen, but i think that getting used to the haldex in a mk1 will be enough to keep me busy for awhile. i literally think that i will need to learn how to drive all over again.
it's good to hear about other people doing AWD conversions. it's the logical progression for the species.