The pictures you have is of a 3/4 ton. The original gears would have probably been 4.56. 3.90 was an option on 3/4 ton later and it was probably installed along with the later engine. The drive shaft you picture is a typical 3/4 ton. There were a couple of years that something like 3.38 was offered in a 3/4 ton auto with a big block engine, but they are extremely rare. How fast do you want to go? The 3.90 will go as fast as it is safe to go with stock steering and brakes.