I would take a look at the school busses on eBay for a possible donor if your want to go V8 automatic. If I remember correctly, they used either a 366 or 427 with an Allison? automatic which would work much better in a truck that large. School busses seem to have low mileage and go pretty cheap. If the body is aluminum, you might be able to sell it for more than the price of the bus.
