I'd just chalk this one up to suspension and gears. The gears in the Z28 make it "feel" faster off the line, along with an older suspension. The Mustang would be faster in a 1/4 mile, but since it only has 3.55s and a better suspension that rides smoother, it won't feel as fast, but in reality it probably is faster. If you are really worried, take them to a strip and run them and see what you come up with. Or dyno the mustang and the Z28 and compare the graphs. If there was an actual problem, that would bring it to light, but just off how it "feels" to you, it's hard to say given they are two different machines.