The "heavy lift!"
I am an unpaid "helper" at a local HDT fabrication shop. In exchange for my floor sweeping ability, coffee making, and general "gophering around" I get to learn all about the construction of class 8 trucks and how to modify cabs and sleepers.
My first assigned project was to take a 379 Pete cab and sleeper and reduce it down to sizes which would fit into a scap metal condola. It took me two weeks, but I really learned a lot. I guess the best way to find out how something is put together is to take one apart!
Now, onto your question. I do not know the size of your ambulance box, nor its weight. But, we regularly lift entire cab/sleeper combinations with a 6K forklift with extended slide-on forks.
When the Morgan dearler placed my 16' box onto the frame rails of my 379, he used the same method. However, at his facility he also had a large overhead crane he used for really heavy lifts, if needed.
Rather than rent a forklift, I would first contact some of your local truck upfitters and see what they have available. I can't believe it would cost more than renting a forklift, plus, they are experienced in setting boxes and securing them to the frame.
Of course, if your mindset is to do it yourself, the forklift with extended forks would work fine; just make sure you do it on a flat, level, hard surface.
Good luck, and please post and let us all know what it went!
Highway OPie (Speed Gray) Grand Rapids, MI: 2003 Peterbilt 379 Motorhome; 550 hp 6NZ Caterpillar C15, Eaton-Fuller 18 speed transmission, 3.36 rear; 63" sleeper and 16' Morgan box.(firstname.lastname@example.org). There are many Peterbilts, but this one is mine!