quick question coz gonna do it now.
got an xr3 mk3 shell putting series one stuff on it. changed the front anti roll bars over but are the back ones the same or do I need to change them. havent looked myself, it to bloody cold

 

xr3 as not got a rear rollbar s1 as

 

S1, 1.6i Orions (83-90) and some late XR3i's (90 spec ?) had the 12mm rear roll bar which is equivalent to not having a roll bar. The S2 had a 16mm roll bar that already makes a tiny difference. The wishbones and mounting stuff are identical for both for except the bushes that obviously have different diameters for the two bars. The mounting locations are there already on the crossrail at least on the never 83-on chassis rail, not sure on the older one.