The 700r4 is an auto. I've thought about manuals but with that many people driving and with some who aren't used to manuals it could be a problem. I looked at the Allison online last night and it looks interesting but if the price is high like you're thinking it may not be the best option.
As far as the box, I think you're right as far as a custom box. I was thinking for the box trucks that I would transfer the AD cab to the P30 frame and widen the front fenders. I may also need to go with a larger wheel/tire or decrease the size of the wheel well opening. It looks like the 19.5 from the P30 would look pretty small.
