454 Dually with 4.56 rear diff, what auto trans to use

[evan love]

hello,

I just bought a truck I've been looking at for a long time and finally knocked on the door and they sold it to me for a great price.

It is a 1979 single cab dually (dooley) 2wd with a 454 and automatic (almost no rust and good original paint, Silverado with great options)

The only issue is it has a 4.56 rear diff, which can pull just about anything, just not at highway speed.

It also has the 10,000 GVW and the camper package.

What I would like to use it for is to tow a flatbed car trailer with a 1973 K10 shortbed on the trailer, and I would like to drive it from Colorado to California.

Which gets to my question...

... do I switch the rear diff gears and lose some pulling ability, or change the 3 speed automatic for a 4 speed automatic, or something else so that I can actually still have the power to tow five or six thousand pounds and still get down the highway at an ok speed and minimize gas usage and engine wear as much as possible?

thanks,

I am open to any and all options with a mind towards low to superlow budget combined with the fact that I do have mechanical skill and tools, help, and access to a lift (though not sure the dually will fit on it)

thanks again,

evan