S197 Mustangs are front-heavy, you need front grip just to get your cornering started, and MacStrut geometry is inherently geometry-challenged. All three represent understeer effects, so all three call for more front tire rather than less. After all, the best way to fix excessive understeer is by adding front grip - reducing front slip angles to be more precise - rather than throwing away rear grip (increasing rear slip angles). Improve the end of the car with the greater weakness first.
You have to remember that you're not working with a Porsche 911 (weight very heavily biased to the rear) or a Viper (big power/big torque and still with a more rearward-biased weight distribution than a S197 GT). Not even a S197 GT500 (meets the big power/big torque criterion).
Using the throttle to crutch insufficient front grip by loosining the tail is not the fast way through most corners.
Adding rake by increasing rear ride height does not change the car's front to rear weight distribution very much. The larger effect probably comes from dragging the rear geometric roll center upward, which does a couple of things (increases the rear percent of total lateral load transfer, and in particular increases how fast this load transfer happens (transient handling on turn-in/corner entry).
What I could find for the FR500 cars (from the 2009 FRPP catalog) was that the FR500S came with 18x9.5" wheels all around, and that the FR500C came with 18x10" wheels front and rear.
I've done a little datalogging with my '08 GT, mostly on road courses but a little on the street. These pics were taken with 18x11 wheels and 285/35-18 MPSS tires on all four corners. The readings are peak g's, which run 15% - 20% higher than the sustained g's through the same and similar corners. I'm pretty sure the car was good for more than I was. Koni yellows, Strano sta-bars, aftermarket LCAs (not poly/poly), and about -2° camber rounds out the mods. Stock springs. Car turned in very nicely on lift-throttle.
And a brief clip from a track session. It really felt like a brisk but relaxing drive out in the country (minus the risk of official-capacity roadside interviews).
I ran comparative tests on one of my unofficial "test loops", running the above wheel & tire combination against 265/40-18 MPSS on 18x9.5" wheels all around. Tire tread widths were listed as identical at (IIRC) 10.2". The car was noticeably more composed and less slide-y with the 285/11" combination at 0.9x lat-g than the 265/9.5 combination did at nearly a whole tenth of a g less.
It's not that the 265 setup was bad, just that the 285 setup was that much better. A 265 front / 285 rear combo would have understeered enough more to clearly feel.
Norm