Pat,
Interesting project you have here. I like the idea of the tandem. yes it was a light to medium duty tractor/truck.however like you said. I am sure someone somewhere did a tandem axle truck. I whole heartedly agree with that theory. Not sure about the Hiab crane though. I think I like the one in the Italeri parts pack better. it is a bit smaller I believe, and also of earlier year.
As for the single drive long wheelbase /long bed. Maybe in the States. But if you go other places they really stretch the capacity of their vehicles. Like here in the Philippines. Some really long drop side and wing vans on single and tandem axles..And trust me when I say. They load em heavy!!!
So I say go for it!!
Be Well
Gator