Cool project although you might upset a few Tonka collectors, but who cares about them. Anyway, Scott (SmallHaul) did a conversion on a Tonka dump truck which might help give you some ideas or at least see how he did things.
http://www.rctruckandconstruction.co...ght=tonka+dump
I think there have been a couple other Tonka conversions on the forum too, just search and I'm sure you'll find them. Hope this helps...