Why is my top speed 70 mph?

HI everyone. I am new here, and I am eager to get into this new project with my son. We bought a 1975 Dart Sport with a newer 318 and newer automatic tranny with a shift kit. The problem is that it is SO SLOW! In 3rd gear it is topping out at 70 mph. 2nd gear tops out at 40-45 mph. It is not even fast off the line. The engine runs great and purrs like a lion, but dang if it is not going to be passed by a moped. I took it in and my mechanic (whom I have lost confidence in) did not see anything wrong. Any ideas where I should start?