Do a beam element test using a program like Algor. Very simple, and you don't have to model everything. Just enter the node coordinates, material properties and moments of inertia of your tubing and you are good to go.
Race car vehicle dynamics has a good writeup on how to test a car's torsional rigidity through experimental means as well. Would be good to do both and correlate the results.
Just FYI my bone stock (oops! K&N drop in filter) BMW 535i (1988) dyno'd at 133rwhp and 150ft-lbs... Actually a fair bit more than a stock 240z/L24, but nothing crazy. It will be a nice platform for future modifications though. With the 8:1CR I am seriously thinking about turboing mine.
I ran a turbo with straight pipe only after it for over a year in my Z... wasn't that loud in all honesty. Not much difference between that and with the magnaflow 3" straight through that I am running now.
8.3:1CR, 6psi boost, SDS, 3" exhaust, 2.5" DP, 60mm TB and NPR intercooler gave me 198rwhp on my dyno dynamics dyno, or ~231rwhp on a dynojet. The increase in CR was well worth it for the throttle response.
If you are interested in one in bc, check this out. http://www.super7cars.com hyabusa powered. Really nice pakage, I've raced against the owner of the company more than a few times!
Sadly I don't have pics from that build. I am not sure why now that I think about it, other than it happened over a very short period of time and I didn't have a camera at the time, apparently I didn't borrow one either. It is NOT for the faint of heart. If you are going to attempt it get the S30 chassis manual with all the chassis dimensions in it and build yourself a solid, level jig out of I beam or large square tubing.