I also recently bought a 73. Due to EPA regulations, along with consumer desire for better fuel economy, our 73's were neutered at the factory. They lowered compression ratios, used really mild can shafts and installed EGR systems. All of that can be corrected if need be.
In my case, the motor was in need of a new carb or at least a rebuild. Rather than simply replace the cab, I opted to go with an Edelbrock performer intake and their thunder series 650CFM verb. It really woke the engine up, partially because the 40-year old factory carb needed work. So I believe my swap gave me a lit more than 10 horses.
I also had a FMX transmission, and once I had swapped the intake/carb, she was a great cruising car... but I still wanted better performance from light to light, so I started to look into a more aggressive rear end. But the problem was, I was already around 2,900 RPMs at freeway speeds (70). So I ended up dropping in a wide ratio AOD transmission, which gave me a more aggressive 1st gear and overdrive.
The tranny swap made a bigger difference in performance that the intake/carb, but it was also about 3x the cist
I suggest doing a lot of research before you decide, and really weigh city versus highway use... then choose what you may want to do.