The term "Show Car" is such a wide brush to paint anything with that you need to be more specific. If you're talking judged shows, such as the indoor shows judged by a panel of experts, then obviously modified will add a bit to separate your car from one with leaf
suspension as it came from the factory. Different isn't always better though, not in the eyes of the judge you hope to impress and not in the execution of the car in general. If it's poorly installed (wheel base longer on one side), designed (hits the exhaust) or finished (orange peel, "dry spray", wavy chrome, or too long of bolts, etc) the judges will mark you down for it, rather than improving your score. Judges balance the overall design of the car, then look at the individual components and modifications. You actually get dinged for using "off the shelf" parts rather than custom made stuff. Indoor car shows are brutal and in most cases the cars are trailer queens out of necessity to be competitive. If that's your intent, you'll need a lot more than advice from a random guy on an internet forum to be in the running.
If you're thinking of the one-day outdoor shows that are everywhere, I wouldn't sweat it. Most times they are participant judged which is why sitting by your car all day shmoozing the public is typically more important than sweating over
suspension choice. Barely anyone gets on their knees to look at it anyway. Nice paint, expensive interiors and shiny wheels with something to look at under the hood work well here. Who cares if it runs poorly or the tires rub, as long as it's pretty.
Somewhere in the middle is the Goodguys-type shows. Typically judged by local car experts, they look for design above all else. If the car looks right, sits right and is unique then you're already off to a good start, but they have so many to judge, your car won't get more than a couple minutes worth of their attention at best. Sadly, Mustangs are a tough sell at car shows because they are so common. Don't feel bad, I love Mustangs, but I spent weeks perfecting the gaps and fit of the aftermarket 'glass panels and the stance of my GT350 clone and people will walk right by my car to look at a '61 Rambler wagon that has a stuffed dog and a picnic table in the display.
I've judged shows and participated since the mid-'70's and I can say with all honesty I'm over it. If you're building a car only to win shows, you're setting yourself up for a life of misery. It's much better to build a car you love to look at, drive and hang out with your buddies with than to spent time and money worrying about what others think of my work.
In the 11 years my car has been driving (they're never really finished) I've won less than 5 trophies period. The car is always spotless, those that do look at it like what they see and I've yet to hear someone fault the car's overall look. By comparison, my wife's restored Corvette and my brother's 1931 tudor sedan hot rod won constantly and neither is of the same quality as my car. I should know since I was heavily involved with both of their builds.
It's none of my business or anyone else's what you do with the car, I just want you to know it's far better to build the car you want, that does what you want and if it wins it's class, so be it, than it is to do anything on the car for someone else's approval.