Auto OD questions

Not "A" body, but I respect many of the trans "experts" on here. Purchesed a 1988 D150, 2 wheel drive, to use as a tow vehicle for my race car. Truck has a 360 4bbl, 727 trans. I'm thinking of installing either a A500 or 518 to improve fuel mileage. My old Toyota PU, would get 18-19 MPG towing, so I am spoiled. I know this truck will not get any where near that. I have both the 500 and 518 core transmissions, that need rebuilding. I have rebuilt many 904 and 727 transmissions, but never an OD. I would like recommendations as to which trans (I'm thinking the 518), and any upgrades that should be done, while I'm building. Thanks.