Why is my top speed 70 mph?

You all are great! Good questions. So, I was told the engine was less than a year old. It looks like it too. Very clean. The problem is that the wiring is all original and is terrible shape, so I will be replacing wires as we go. Right now I am having intermittent problems getting spark due to a wiring issue somewhere. Anyway, the engine revs well, and the carb is open. It sounds like we are doing 120 mph, but yet, we struggle to hit 70. I am thinking that we do have a gear issue going on. Maybe they are 4:10 gears in it? I don't know. Any way to know without taking the cover off the rear end? Even then, I am not sure what I am looking for or what I am needing to replace it with. Basically, I just want it to not be revving high at 40 mph in 2nd gear before it goes into 3rd, and I want us to be able to hit 80 with plenty of pedal left. This is a hodge podge of a car from several people before us who did some things well, and others not so well. I wish I knew the history of the car and could talk to whomever did the work over the years, but I cannot. So I will be learning as I go on this one. BTW - - my mechanic I took it to to give it a one over said it had a shift kit on it because I was concerned about the hard shifting.