Nice car mate. I spoke to your Dad at Buller a couple of years ago after the last stage. If it WAS your old man, I rate him, what a character!
80mm ride height might be ok, I've never actually measure my actual ride height. Spring rate wise on a typically rally weight 33R I'd be looking at 600lb FR, 400RR. If you like a stiff feeling car you could maybe step the front up to 650. That's my own personal opinion/preference. Other guys will tell you to run much stiffer, in the rear particularly. Personally, I like a car more friendly to the bumps. Still, you need to ensure you're not bottoming out on the big ones..... and therein lies some of the challenge that is Targa