The biggest issue with the 128 chassis, and tuning it to get the tyres to work, is chassis flex.
No matter what you want to do with camber curves and setting the car up while it is static, this all goes out the window in the real world, especially if you also have an engine that produces considerable torque.
The front ends flex, adding a bigger / stiffer front bar just makes it flex more. There are guys in eastern europe that make a "kit" to reinforce the Zastava, but with all the bolted in attachment, I seriously see only little benefit from most of it. My rally 128 had considerable plating and undercar re-inforcement. The standard chassis will flex enough to eventually develop serious chassis cracks when you improve the engine output considerably.
same with the rear roll stiffness, the lower inner bushes of the control arms have sufficient flex under hard cornering loads to alter the rear toe. I also had the issue (in rally) of bending the rear lower arms and the attaching spindle bolts. My solution to both issues was to add a forward brake reaction rod (a shortened 128 coupe unit) with the forward pickup point mounted so it was aligned the other two mount points (on the lower arm) so as to not cause rear steer with suspension travel.
All the well prepped racing / hill climb 128's in Europe I have seen mount a lighter , slimmer bar solidly mounted on the firewall (more or less under the steering rack) with adjustable driving links for attachment as close to outboard as possible on the lower arm.
View attachment 40095
this is one of the kits available from eastern europe, I don't know what they did with the sway bar in this pic, but looks likethe replaced it with some forward reaction rods with a ball joint / knuckle at the forward end.
View attachment 40096
this it the kit... as I said with all the bolt in attachment, I don't see a lot of addition stifness, but it would do something.
SteveC