That's not a very popular configuration because of the drivetrain issues it introduces.
Is there a reason you aren't considering the bakfiets design? Lots of help, plans, and kits there.
It's fun looking at pictures of cargo bikes. I see that putting 2 wheels on the front is much more common than the rear.
https://www.google.com/search?q=carg...w=1288&bih=668