The easiest would be a later 235 or 261, virtual bolt in. Any V8 is going to require modifications to the frame and steering system in an AD truck. Of the available 350s, the Chevrolet would be the easiest and lowest cost. To get around the steering box issue, there's a few ways to do it. One would be to relocate/replace the box with a later or smaller box, and use a late model/aftermarket steering column. Another, if you want to keep the stock box, is to relocate the box using a spacer to offset it to the left approximately 2 to 2 1/2 inches.
Due to the extra width, a Cadillac engine, as well as the Oldsmobile and Plymouth 350s would not be desirable in an AD chassis, a Buick 350 might work.
