I'd figure out what kingpin pods were necessary to install IH or f700 10 lug spindles and hydraulic brakes on the front
Your axle beam is going to be narrower than and f600 front beam.
And for the rear I'd use an IH rear axle with hydraulic DISC brakes...the ford hydraulic drum brakes can cost a grand just for a pair of loaded backing plates if your wheel cylinders are bad
Engine and tranny? look for a cummins or IH444 out of a B600-B700 school bus or ford? international commercial truck.
The 8 liter cummins ford put into their f700 and f7800 dumps was a good engine..the 5.9 cummins was not just used in dodge pickups but also in ford 2.5 to 5 tons
The International DT466 engine is a really good engine too
if you build it with modern commercial hydraulic brakes instead of air brakes it will not need a cdl
If you build it with a diesel it will not be crippled by excessive fuel consumption
If ypu build or intall a commercial tilt and slide or rolloff body or even 'arm and hook' flatbed or container you will be building a truck that can sell for 20-30 thousand or more if you do it right and built it DOT legal.
If you stay with the legacy brakes it might not meet DOT requirements. any brake system leaks = out of service.
The old fashioned air brake setups also are not in compliance with current DOT regs
A couple years ago I saw a bus on Ebay with a 5.9 cummins, 60k miles, 22.5 aluminum wheels with new tires runnign and driving for $3,000 buy it now.
I had an 85 f600 with a gas engine and a 22' flatbed I was looking to turn into a diesel with hook arm, removable bed and a winch
if you think the tilt decks are cool look t a hook arm setup...
you can drop the whole bed on the ground and have a couple different beds for the one truck.
you could even have a flatbed you could drop to pickup trash containers.
I saw a dump truckk with a bobcat in the back one day and asked him how he got the bobcat up in the truck he told me he just uses the arm to put the whole box on the ground then drives in on and picks up the whole box.