Dave you can make a sedan in to a sedan delivery pretty easy if the sedan is a flat back sedan and not a (humpback) trunk style.
you could weld in the side windows and then make the flat back trunk and rear window a door, you will need to make a upper door frame.
see the pics to see the difference between the two.

now as far as the 39-47 trucks go
I added 3" to my cab behind the door on my race truck.

you could add to the cab behind the door
AND lengthen the door like 4" each to both and have a cab 8" longer and I think you would fit just fine.

EDIT:
Dave I have been looking at 35 pics and I don't see any "flat back" sedans with trunks, so if you did a sedan delivery you would need to build a rear door and frame OR just leave the back closed
(not my favorite I like a delivery with a rear door)