I'm considering something similar with the 1061 trailer, but making it a gooseneck that has a changeable hitch (a 4" tube) inside a sleeve that is 5". Initially it will hitch to my 931, but could just as easily attach to a deuce or be changed to a ball (2 5/16" or 3") gooseneck. That way it doesn't need a tractor but would readily adjust to about any pull vehicle. Has air over hydraulic brakes.
The pivoting axle on the 1061 will make it so I can tilt the back, the original trailer, for loading and then flatten out to drive machinery further forward...like transporting a telescoping boom man-lift. Simply adding 12 or 14' and keeping it flat or dropped deck, would be a good way to go if loading was not an issue.
My M35A3 arrived today and limped into the yard (off the truck). It has tranny problems but with only 10k miles has potential. It gets new 395's XZL Michelins and TLC with the Allison...and paint. Sure would be nice to dump.
The front of your truck is different than mine, the thinner section in the middle (bumper) is up on mine and would be really difficult to change, even though I like the look of the double bumper, matched like you show them.
A word of advise. Don't make it so custom that you have too much to do and can't get it done. Do you really need a fifth-wheel or would a bed with ball hidden beneath a plate work for your needs, but retain the use of a bed. A 2 5/16" ball gooseneck can have up to a 30,000# trailer and a 3" can do 40,000#. Then if by chance you did want to sell what you build, you have not made time and money consuming changes that you don't recoup your investment. Then again if you already have or have a lead for a deuce tractor....maybe that is the way to go. I had a cargo in mind.