what you want to do to achive greater lock is to move the pivot point of the tre closer to the hub
Shorten the arm on the hub where the rack bolts
I said that
Shortening the hub arm shouldnt be too much effort, if anyones any good at welding cast iron.
Easiest way to find out would be to try it, sierra hubs and whole cars are cheap as chips, and there won't be huge differences in standard to xr/rs stuff.