Its time to play prod saloons!
As a Prod -Saloon our Suds were NOT allowed rear anti roll bars, so to increase the rear roll stiffness you have two choices, or ya can do both (We did!
FIRST! Very early suds had shorter trailing links on the rear suspension, which means they swing through a tighter arc and therefore increase the roll stiffness. If you can find a pair simply cut off the mount on one side, fit the shorter arm, with the mount attached and weld it in the new location. Repeat for the other side!
NOW! If the eight rubber mounts are replaced on the leading and trailing arms with solid nylon bushes, again the roll stiffness increases. You can fine tune the roll stiffness by using 2, 4, 6 or 8 of the lil buggers! THAT gave us 5 settings to play with!
8 solid bushes is probably too many and you loose all the "give" in the system. Try it with just one on each arm in the first instance.
We would take 3 complete sets of arms when we went testing so we could mix and match to get the best setup, but this was nearly 30 year ago so dont ask me what we ended up using, now!
Gotta be cheaper than that A/R bar!