Before you bust the budget, I could offer you my friends MULE for $5500 and you'd save a H... of a lot of time and sweat equity and have SO much more fun getting looks from just as many people with SOooo much less effort...
Just a thought.
I could use the airpac off the rear winch for my 543. One day I'll have to pull the boom cylinder to replace or tighten the seal, she leaks like a flood when fully extended. No access plate to check and tighten the nut on the end of the cylinder either.
By the way, have you downloaded the 5T manuals for the M543 before you look under the hood? the -20 is mandatory, and the -10 is a given before opening the door.
As to the concept of pulling the bed and having a neat old truck, not a good idea IMHO, as it's underpowered, and geared to tow another truck coupled to it, therefore you will still have a very s-l-o-w drive up to 45mph, then eventually hit 50-55.
That Mule's looking pretty good right now, fits in the back of a pickup truck ya know... (and has a carb)
V/R W. Winget