Mustangs are actually a poor choice. Their handling characteristics are significantly different from that of a Viper. Mustangs, especially that '95, understeer. There are two basic behaviors Mustangs (at least that chassis) do not provide that Viper would need to learn for a Viper. First, they do not exhibit throttle-off oversteer the way a Viper does. It also doesn't punish poor combinations of steering input and increases in throttle the way a Viper will. Believe it or not, despite the significant differences, a Miata does. So does an S2000. The old Brittish roadsters will, as well.
A friend of mine started his track time in a Mustang, of a similar chassis to that '95. He didn't understand all this talk we had about throttle off oversteer, rotating through the corner, etc. He asked me to help him one day, so I hopped in the right seat, and we went around the track. I found a number of bad habits his Mustang allowed that the cars he was considering upgrading to wouldn't. I tried to correct the behaviors, but he was unresponsive. So I stuck him in the left seat of my Miata. For the first few corners, I talked him through everything he needed to do, helping him avoid the bad behaviors. After giving him a chance to learn the correct way, I picked a slow, safe corner and stopped talking him through. All his bad behaviors returned, and the car went into a rather nasty spin.
Imagine if that had happened in the street in a Viper instead of on track in a Miata.
Going straight to the Viper, and keeping it in high gear with minimal throttle inputs would be better than going to a muscle car balanced toward significant understeer. Keeping the Viper in high gear will reduce the power it can actually put down and will reduce compression braking, making throttle off behaviors less extreme. This means the car will still punish bad habits, just with a less severe punishment. You need poor habits to be punished right from the start, so you don't develop them and then have to un-learn them.