What year/Model is your car? I hnow you aren't talking about the 02 GT.
One reason is technology. Newer cars put it to the ground more efficiently. Doesn't mean that they can't be made to be fast, but it takes some real mods. Your stock 302 is probably hovering around 200-220 HP right? Going through an AOD and the rear end, that's considerably less than 200 HP to the ground. To put that into perspective, your 02 GT (stock) should be well over 200 HP to the ground, and be somewhere in the mid-high 13's.
Another reason is traction. It takes some special voo doo to get the classics to hook up. Not impossible, just another challenge.