I have a small hauling business and do some dirt work, we added a hooklift truck to our fleet a couple of years ago and it was a great addition. I bought a used ampliroll hook (60k) on e-bay and mounted it on a quad axle 357 Pete. The guys at Marrel have been very supportive when I needed technical assistance.
I run 10-40 yard cans. Seems like the 20-30's are most used by my customers, as mentioned before the 40's get overloaded too easily and I tend to use them mostly for my own projects. One thing I did recently that comes in handy is got a 20yd that fits inside the 40, this saves a trip when going to the demo site and I use the 20 for scrap iron or something else that may need to be sorted.
I also have a flat bed for the hook lift that is very versitle.
Although my Pete has a 9ll trans I think an allison would be the way to go if you can. I have a couple tandem KW's with allisons and think it would be really nice in a hook truck.
This winters plan is to re-mount the hook on a bit newer KW chassis that we built from a wreck. I will post pics when and if we get this done.