The Sapphire 2wd one effectively lengthens the wheelbase on a 3 door with the original ARB.

They designed it so that it would give about a degree more caster effect on the 2wd.

I think some of the later 3 doors got them standard anyway ( like the RS500s ) and it was available as a retro fit option on the earlier 3-doors from the dealers if
a. the dealers were savvy enough
b. the customer complained of poor straight line stability.

How much clearance did you have at the front on the original ARB with your 8x18 wheels?

If it was very little the clearance might be tight as the later ARB will sit the wheel further forward in the front arch.

If you want the extra caster and you want to retain the big wheels wheels another alternative to comp struts/909 ARB is to use the top mounts that Mike R did a run of ( the downside being you will have to cut our your strut tops ). That will tilt the top of the strut back rather than push the bottom of it forward. I'd check it will work before you try that one though!

Just an idea......

Anyway, hope you get it resolved