However you decide to do this, please take lots of pictures and keep us (me) posted. I'm planning a similar project. I have a '59 short/narrow, a '55.2 panel, and a '77 short/narrow to work with. I want to take about 6" to 8" out of the front of the bed and extend the cab about that same amount. I don't need a back seat but just want to add some leg room and a little storage. Here's a site that sells kits for the frame swap.
http://www.e-zchassisswaps.com/1955.htm
I haven't purchased one but have it bookmarked for when I get to this project. I'm still undecided about whether to use the '77 frame or leave it on the '59. I won't get time to work on this for some time so I'm still just planning and gathering info now. Good luck with it.

Jeff

'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc