255/45-17 is a decent size tire. It is way bigger than anyone had back in the 70's but way smaller than what BMW or Porsche engineers would put on a car your size these days. So you just have to decide how "modern" you want to be.
A 17 inch rim allows you to run a 13 inch brake rotor which should be big enough to stop that car. Do you know how much the car weighs and what the weight distribution is? If you haven't had the car scaled then that should be a high priority. You need to know what you're working with before you start working.
I assume the car is at least 4000 lbs and probably has 52 or 53% on the front tires. So there is a lot of weight up there which means the tires are getting tortured. A 255/45-17 tire can handle around 1500 lbs of load but that doesn't mean it is going to handle great when it is fully loaded.
Since you have a running combination you should be able to crawl underneath it with a tape measure and figure out if you can put a bigger tire on it. If you can great, if not then you know what your limitation is. If you are limited to a 255 section width then consider a bigger wheel. Perhaps a 255/40-18 or a 255/35-19 would work better for you. A 19 inch rim would allow a 15 inch brake rotor which is what the Porsche guys would use these days on a 4000 lb vehicle with 500+ horsepower.
Go spend some time on the BMW website and look at the tire sizes and brake rotor sizes they are putting on their 4000 lb cars. Or visit your local Dodge dealer and look at what the Dodge engineers put on the Hellcat Charger. That is a big car and they put big wheels, big tires and big brakes on it. Once you have the correct size wheels, tires and brakes then you can fine tune the spring rate and shock valving.
You'll also need to stiffen up the chassis a bunch. Ma Mopar made these cars as limp as a noodle back in the day. XV is out of business but you might be able to dig up some of the stuff they published on chassis stiffness. They were the only business to ever take the subject seriously but the market must not have been big enough to keep them in business. Basically you'll need to add some crossmembers and a monte carlo bar.
Awesome response, thank you!
I do need to get that car weighed. I've always assumed it weighed around 3500 lbs, based on nothing more than old Car & Driver articles and the fact that I have aluminum heads, intake, and lighter front seats!
When I first moved to SoCal I assumed there'd be a huge drag / SCCA scene here, but as far as I can tell there's not too much due to the high cost of real estate gobbling up places where one might try to race. I think I can weigh the car at Irwindale.
I do wish XV was back in business. Before I had any budget, all I was able to afford from them was their seat belts, which I still use and which were well designed.
I also looked at the Corvette C5/C6 suspension site posted above, but you need to replace your entire rear frame to make that work. I want to keep the car somewhat original, so that's a no-go.
