I believe a 273 would be a more fuel efficient, driveable package. A lot of the slants did not get any better fuel mileage than the 318 vehicles in the day, especially in heavier vehicles.
I had a few newer 318 magnum pickups that got 21-22 MPG, I don't think a slant in a full size pickup would approach that mileage, especially in the mountainous terrain I drive in.

Sheldon