Hi Gary
To be clear, I believe you to mean the suspension arm which runs directly across the car from the ball joint at the bottom of the stub axle to a rubber bush joint on the chassis leg. About 1/3 of the way down it is the attachment point for the arm which runs diagonally back to the chassis leg.
OK. Well the 2000 arm is longer than the 3500 / 2200 arm. If you want to swop models there is some point in putting a 2000 arm on a 3500 as this will give negative camber to the stub axle. The other way round would be a disaster - remember what a Triumph Herald looked like after the swing axle had jacknifed and was running the outer tyre on the sidewall....?
Either way there is an unquantified problem with the diagonal lower arm which is likely to need cutting and shutting to restore the correct vertical alignment of the stub axle. Perhaps there's even some scope for introducing some anti dive geometry by leaving the stub axle slightly inclined back....? (effectively lengthening the wheelbase) There might well be problems of a similar nature with the steering side rod, although I need a week in a dark room to think that one through!
IE amateurs had best give this one a miss!! But if you fancy a few months of happy experimentation you could well come up with a more effective geometry than the factory did.
Hope that doesn't either discourage or overely encourage!
Regards
Chris York