Are we talking about a dedicated track car or a street/HPDE/auto-x car?
A street legal HPDE car, but not a daily driver. I've given up on autocross. Way too much time spent not driving, class creep with the newer technology... blah, blah, blah.
A n/a 331 in a fox can easily get you to the wt/hp ratio of a boss S197 or 5.0 s197.
Maybe with a non-stock block. But too many stock-blocked track cars have cracked the blocks at relatively mild power levels. Then again, I guess a 331 stroker shortblock built in a decent block, a lot of used parts and the cost of swapping to a 351w based motor might be largely offset. (Though I'm already in the middle of swapping a 408w into my car.)
Two things I would ditch off your list are the IRS and 351, both carry a hefty weight penalty and that's about the only advantage the fox has.
And yes, I would lose the IRS and go with a 331 to take advantage of the Fox's light weight.
I'd referenced that other post comparing a prepped IRS to a torque arm car, and it seems that even with the extra weight the IRS is superior. My preference would be toward trying the IRS - less sprung weight, and even though it is a little heavier, it's low and in the back over the axle line. I do realize that most still prefer a non-quadra-bind solid axle, point taken.
The fox can be made faster for cheaper, IMO. But the tradeoff will be it's streetability/reliablity. The S197s will be a better platform and require less changes to make them as fast but you're starting at a much higher price point.
Meanwhile, if I take the plunge and get a new GT or Boss, I'll be afraid to push it because it's too expensive to break. I also won't be drilling or welding on it, so the race seat and harnesses my Fox has wouldn't be in a new car. It's harder to drive when you're bracing yourself through corners instead of the seats/harnesses doing that for you. Though the steering in the new car is worlds better than my Fox steering.
This is sort of what I was getting at, and was wondering how the two chassis stacked up. I certainly didn't (and don't) want to start a "how to make a fox handle better than a ..." thread. Just a curiosity as to how far some chassis stiffening, better weight balance (IRS and forward offset MM front), and decent suspension geometry would take the car.
The new Mustang seems to be the pennacle of Mustang performance, but with the cost of the new cars you may as well be playing with a used C6Z (particularly against a new Boss).
This is all mostly a mental exercise though, as I don't think I've any serious intentions of getting my car this far – I'll soon be back in school full-time for 3 years for a master's. It's damn near 100% that the Mustang will get sold at some point along the way, as I doubt I'll have the money or the time to do anything with the car. Assuming it does get sold, I'll probably not get back into Mustangs when I'm back to that point financially. I think an early air-cooled 911, any number of Factory Five cars, a used C6 Z06, or even a LS powered 996 (Renegade is working on a kit) would be more fun.