Very good questions and I'm glad you are considering it, the rear brake is quite important imo. You are dead right, the bolt/mount you have on the Heckler that mounts into the frame should pinch tight but many people just couple that up to the lower 'swingarm' and I guess it is forced to move, being ali is probably becomes a soft bush! It should mount to the brake boss and not move, I think it is all from the LTS...
So the IS mount should remain static but I am going to bore it out so it will take the plastic bush, use the frame bolt and couple it to the swingarm like the Hope C2. (btw email them and you can get free samples if you pretend a bit)
There is an issue of disc size, 180mm will fit but using the IS mount you can't actually connect the torque arm because the disc will be too large. Disc needs to be 175mm or less from memory, certainly 160-165mm is fine and what I plan to use, along with a Mono M4 number 3.
Check this out, lots of pics n info: http://www.retrobike.co.uk/forum/viewto ... highlight=
97 GT LTS-1
99 GT Lobo DH 1000 x 2