8flat,
The 1-1/2/2-ton trucks are different than the 2-1/2 ton, the frames are bigger as well as the springs. The fenders (larger wheel opening and flare) and grille are also different (extra opening below the grille bar). Take a look at my pictures in Mike's Garage (link in signature below). The blue farm truck is a 6400 and the Tow & Fire trucks are 2-1/2 ton.
The 10.00 x 22.5's will work fine on the rear, but they will rub on the front in the '56 (they work on the '57 and up front as the front axle is 2-1/2" further forward).
If you are going with the BB you will need to install new side motor mounts as I don't believe the BB has the bolt holes for the TF style front mounts. With that said, you can then remove the existing bell housing cross member and add a new trans mount and place the engine/trans where it works best for you (forward and to the right). You can also move your radiator from the V8 position to the 6-cylinder position which will give you more engine space (5 inches).
The 454 won't even break a sweat moving your truck, but it will like gas! I'm thinking somewhere in the 5-6 mpg range, loaded or not.
Mike B
