Finally got the nut off! Took your advice Hoodeng and made up a sprag plate, it worked a treat. Had to buy a $30 butane torch from Bunnings to heat the nut a bit, then the 1.5" single hex impact socket on a 1/2" drive 600mm breaker with 3 feet of metal pipe cheat bar (trolley jack handle). Thanks for all the advice guys, very much appreciated.

Also does anyone know of a good Comp assembly replacement for a 1990 fxstc? Suitable eliminator kits only start from '91 onwards apparently.