Newbie Question, Need one more gear....

[bmwhitt]

I searched but couldnt find exactly what I needed. I recently bought a 1964 GMC 3/4 ton with the 305 V6. I have been driving the wheels off and love every minute of it. It is lwb and has a utility bed on it. My question is, what is a direct replacement for the rear end, gears, or maybe a 5 speed trans that bolts to the big V6? The motor seems to be screaming at about 45 mph or so...( my speedo isnt working). This was my grandad's truck and dad said it would pull a house down if you could keep the a$$ end down. I just want it to be a little more drivable maybe 65 mph or so.....Thanks ahead of time and sorry if this is a repeater!!