OK, ignore the under-informed question above. I went back to SCCoA and reread those threads, plus several more. Now I think I understand that Level 5 is DSS halfshafts with 31-spline inner and 33-spline outer ends, which require special hubs. Level 2, then, must be 28 or 31-spline inner and 28-spline outer ends. Both would be stronger than what I have, but Level 5 is the strongest possible IRS setup.
So, back to my original inquiry. Assuming I can resolve the high IAT issue (working on that now) and get more timing advance into the tune, I could end up with ~400 RWHP and ~450 RWTQ. The car is never going to have nitrous or a transbrake, and likely will stay on DRs. Given all that, do I really need Level 5 or would Level 2 be enough? The combination I'm thinking about upgrading to would be a Detroit Truetrac and DSS Level 2 shafts, both 31-spline. Will likely go back to 3.73 gears at the same time. What do you guys think?
1997 Thunderbird Sport
5.0L-2V PI S/C
540 RWHP / 492 RWTQ
11.28 @ 123 MPH