Don't know much about the conversion, but as an owner of a 2000 F350 Diesel Dually, I can tell you the fenders appear to be pretty simple plastic 'add ons'.
But for plastic addons, They seem to want a pretty penny. Hopefully you could find the used fenders at the same place you found the rear axle.
You would also probably want to make sure the gear ratio's match, if it happens to be a 4wd. I know I had a selection of a few different ratios at the time I ordered my truck.
Programmer - An organism that turns coffee into software.
If all your friends are exactly like you, What an un-interesting life it must be.
"A door is what a dog is perpetually on the wrong side of" Ogden Nash