You will have to either remove the outside set of duals or block them up the sit above the rail of your trailer. If you use a deck over trailer then width won't be a problem as the bed will be a full 8' wide.
Most non-deck over trailers will not be heavy enough. It is not very common to find a 12000lb trailer that is not deck over. You have to figure 8500lb+ for your truck and 3000lb for the trailer....once you find a trailer rated heavy enough you may find you have no way of pulling it.
I've moved a couple '40's KB8's with my truck and trailer. I have a cronkhite trailer that is rated at 12000lb and is not a deck over. The front tires hit the rails on both sides and If it were not an 18' trailer I doubt they would have fit on there length wise. My trucks had dayton wheel so I could not remove the outside dual so I blocked up to get on top of the 3" or so rails. Any wider and the front wheels would not have fit between the fenders.
As to what to pull it with I would say you definitely need a 1ton. Mostly for the dual wheels to give you stability and enough capacity for your tongue weight. If you figure 10% of 12000lb you have 1200lb on your rear axle...That is a lot for anything under a 1ton. As to fishtailing that is a simple matter of weight distribution....too little tongue weight and you will fish tale. A heavy tow vehicle will help with a poorly loaded trailer as the trailer will not push it around and make the problem worse but will not eliminate the problem altogether.
Let us know how the move goes...and we all love pictures
Tad
