If I were you I would email Lane Kagay of Cetma Cargo bikes. He doesn't just do cargo bikes - he started out doing and still does hoss front racks for regular bikes. He would likely have a good recommendation for a regular frame that works well for hauling in front as that is part of his prime buyer market. He is a nice guy and helpful. His cargo bikes rock too, but they are pretty specialized. I think that Lane is an occasional poster here as well. Lkagar is his user name IF I am remembering right.

Good luck.