Rear arb's are xr4i 10mm, 3 door(cos) 14mm, 2wd sapph 16mm, 4wd sapph 18mm, escos 22mm.
There can be clearance issues with the bolts, it is possible to fill a std diff mount with poly which works just as well, but you know it fits 100% perfect even with a 22mm escos roll bar as the bolts are recessed, to fill costs around £25, Ikotarin from pf is using one I did for him on his 3 door.
AS you rightly say if using a sapphy arb you will need to weld on mounts for the arb brackets on the section above/behind the diff, as well as the arms, alternately could fit sapphy arms.
The brackets are the same on all sizes of arb on sapphy, or you could always have a custom one off arb made that locates exactly the same as the 3 door one, if you want standard looks, just made thicker.
Flexing of the car ?, this is mainly down to the softish(in performance terms) shell, the mk1 seem a lot softer/wobbly than the sapphy, adding a rear centrally bridged rear strut brace will help in this dept.
The rear arb if 22mm sapphy type will bring a whole new feel to the car, to coin a phrase I heard on TG, "I've never driven a car where the rear doesn't seem connected to the front", this is how I view the cossie the front is fairly planted the rear is all over the place.
For an unbiassed opinion ask opposite lock or sir nose as they have been in my car.
I always ask myself would I do the same mod(whatever that may be) again, if I can honestly say yes I'm happy, if no, it didn't work as far as I'm concerned.
Would I do it again, I'd have got a poly diff mount and a 22mm arb before I even bought the car.
These are just my views, I am already on coilys which help a lot imo, opposite locks mate said he'd never been in the rear of a cossie so comfortable(he has one himself), a few people have said this before, other just scream ha ha!!
tabetha
tabetha