rear axle ratio

[chevytrucks79]

Where are the directions to do this or how do I figure out my rear end ratio?

I have a '69 GMC Custom Camper. I am wondering because the other day I pulled a trailer (about 3 or 4K lbs) and the truck pulled it well but it basically topped out at 55 mph. I still had room to acellerate a little but the truck sounded better at that speed. I also got crappy gas mileage, about 8 mpg. My buddy, by guessing the way it drove, guessed it probally has 4.10. It is a heavy half ton from what I know. It has 6 lug wheels, 12 bolt rearend, 350 engine with edelbrock carb and intake and 350 transmission, rear leaf springs. The truck handled well with the trailer, I was actually surprised. I do have a little bit of play in the steering which was amplified with the load. I am going to add a front sway bar this weekend as well as overload springs for added strength and because the rear end sits slightly lower than the front. I figure this will raise it up a bit. I also want to sell my carb and get a quadrajet because I think it will improve my gas milage overall. I put new springs on it a couple of years ago and rebuilt my steering system.