[5Tractorguy]

In a big truck myself, I'd stick with the manual, think you'd have a little more control when hauling loads.
There has been talk of guys wanting to put the 4LXXX transmissions in larger trucks. I don't know what the capacity of those transmission are though. If you dig around on here you may find the info, or someone else will come up with it.

If you can find a diff carrier with a higher ratio that can be swapped out too.

Some of the C60's that were used for fire trucks had the autos in them. I don't know what they used for a trans off hand though.