The flat gaskets seem to work fine but you could certainly modify the headers and flange to use a doughnut if you want to.
I think the muffler on a stock truck was pretty much aft of the cab. Someone with a stock ArtDeco could chime in here.
What I did on my AD was to move the mufflers up under the cab, using a flex support at the rear of the mufflers. Then a longer tailpipe, exiting out the back with another flex support. I think the longer tailpipe give you a better sound with the duals.
