I don't know much about plastic gas tanks. Is this a backup tank type option?
I thought all these D-150's had either a standard 22 or (my long box has) a 30 gallon tank. If it was plastic OEM, I'd imagine what you'd be finding for aftermarket would be metal. Do you know how many gallons it is?
This little guy says it will fit your year, and is a fairly small tank that looks like it would mount to the frame. Is it anything like what you need?