71 302 dieing?

i have a mustang with a 302 2v motor in it. i want to know if the motor dieing or not. it has long tube headers and true duel exhaust. when i feel the tip of the exhaust pipe on the passenger side it feels like there's little exhaust coming from it where as the drivers side has a strong flow. it has new single chamber mufflers on it. the car has 123,000 on it as of now. the other thing i noticed was around 2,250 to 2,500rpm it has a vibration to it. doesn't matter what gear im in. it does have a c4 trans. past that rpm its smooth. the car idles very well and sounds strong. also i don't think it has factory gears in it anymore cause at 100mph its at 4,600rpm. is that correct of has someone changed gears?

The first thing you should do is a compression test of all of the cylinders. It's best if the engine is warm for this test. They should be within 10% of each other. Do you get the vibration only when the car is moving or do you still get it when you bring the rpm's up in park? I'd be checking the engine and transmission mounts as well as the harmonic balancer in a 40 year old car!