Car: '70 Dart, 225 super six, auto

The longer I own this car and get used to how it drives, the more I am able to notice how the various improvements (or changes from stock if you prefer) have changed it's performance (for the better). Overall though, I still am having a hard time figuring out what a really dialed in, reasonably stock slant should perform like.

I have noticed that when I am driving on the highway and already going 55 or so, if I floor the gas say to pass someone, I get nothing but a sound that seems like the engine has more to give but something is holding it back. On the other hand, I can build up continuous acceleration slowly unitl I am going faster than I comfortably want to, at least until I come to a hill. Then, on the hill, no matter how far down I press the gas pedal, it will only give so much.

Are these just symptoms of a three speed transmission and "highway" gears? I know the slant has a low redline, and I have driven stick shift cars my whole life until this one ( so i'm used to being able to downshift at will and run up the RPMs as much as I want), but I can barely even get it up past 3000 rpm even if I manually shift it into a lower gear on a hill, which is not something I feel is a good idea.

What I would like to achieve is a car that is fun off the line, has plenty of power throughout it's RPM range, and can cruise at 21st century highway speeds easily. That's not asking too much for a car built in 1970 is it ?

My winter project is going to be installing a front Dutra manifold and modifying the rear as per the instructions on this site and putting a dual exhaust in, but I fear that that will only go so far and that an OD trans with deeper gears is what I should be thinking of next...

Any thoughts or similar experiences/desires/solutions would be welcome.