Not really all that hard to say, the giant big block motor has a ton of torque and is more powerful than our reasonably efficient small/modern motor. There is no replacement for displacement. Stock the car is slower but there were guys back in the day running 11s in those cars by doing
suspension work and uncorking those big blocks.
Here is some interesting info, the 1/4 mile time for the 69 is from an auto trans car and it ran 13.9:
https://web.archive.org/web/2012100...om/musclecars/general/musclecars-50fast.shtml
Our Machs curb at 3,380 (
https://en.wikipedia.org/wiki/Ford_Mustang_Mach_1), the 69 Mach came in at 3,469 with a SCJ (
http://www.caranddriver.com/reviews/1969-ford-mustang-mach-i-review). There isn't much of a weight difference even with the huge motor (less than 100 pounds). The smaller motor old cars are actually lighter than our newer cars. If you read the article from car and driver you will see they have nothing but traction problems. A small amount of
suspension work and modern tires would fix that and it should shave a TON of time off the 1/4. Basically in stock form, the car can't hook or use all its power and that is why they are as slow as they are.
Do I want a 69 Mach 1 SCJ car? Hell yes. Would I rather drive it daily over my 03 Mach? Hell no. You are right, the modern cars are more comfortable and efficient. The
suspension/chassis design is better on the newer cars but when you come down to raw power, the old SCJ motor is a beast and it would kill our lovely little 4.6 v4 motors if it had the chance. Now you start modifying both the motors and introducing forced induction and such and everything goes out the window.