Sway bar fitted, and early impressions on a two minute drive swinging down through some smooth(50km/h) bends, and on a dirt road, is that the car feels more precise and taught, basically improving on the feeling I had when the gas shocks were fitted.
It's actually feels like more of an improvement in the rear end, which now follows the front exactly...that's the best way i can describe it. Before, it was as if the rear end was a heart-beat behind where the front of the car was going.
Not the most descriptive report, I know, but it's much better than before. The guy at Pedders (Bathurst) who reckoned it wouldn't make much of a difference with a leaf-spring rear end was completely wrong.
Anyway, will take the car for a decent drive on Moday on some of the bumpy 100km/h corners where I first found out about the car's soft, slack suspension and dodgy handling, and I'll write something with a bit more meat.
Once again, thanks to everyone for the help and advice; much appreciated.

I had once fitted a clamp-on style rear sway bar set-up, and found after a couple of months, that the U bolt clamps would slip around on the axle housings.......so the next time I fitted one, I made weld-on mountings for the sway bar D bush brackets.......problem solved.