I have a CMC Widebody with stock front beam and rear torsion bars from a 1969 VW Sedan. I run 16" Fuchs and have had 6" front and 7" rear rims. I would have to pull one for the Porsche part number to get you the dish and backspacing on them (that's probably important).
With a 7" wide rear rim (225/55 tires), I had to use a 1-1/4" thick hub spacer/adapter to push the rim out to fill the wheel well. That adapter took me from a 4X130 VW lug pattern to a 5X130 Porsche pattern. Got them from Pelican Parts. There was probably another inch to go to the fender lip, so my guess (and that's all it would be) is you could fit at least 8" rims back there (front, too). You could also get adapters from VW to Chevy, and that would give you more wheel style choices. I've never had a wheel bearing failure in 20 years running this set-up. You can also use Porsche 924/944 rear diagonal arms/hubs and spring plates - Not sure if the torsion bars have the same splines or not but they will be stiffer on a speedster replica (it's lighter).
I have no experience with anything larger than 16" wheels, though.
I agree with Wolfgang that the only way to really tell is to pull a wheel, then measure from the face of the brake drum back to the spring plate for that inside distance (Don't forget to subtract something for the tire sidewall), and then again from the drum face to the fender lip (again, subtracting something for the tire sidewall). That will give you both the overall rim width as well as the amount of dish needed.