Well one plus is belt tensioners are available for them.
I've driven a couple at work and a friend has a 6.0 in his 69 Jeepster Commando. Strong motors for sure and they don't have the injector issues like the 7.4. I've heard of some finicky dash controls and electronic gauge illumination issues with those trucks but if you look hard enough you can find something wrong with any truck. It's what makes em trucks right? If the truck is clean and well maintained I'd go for it.