Not sure what body style you are working with - In any event take a tape measure - and measure the distance from the outside of one leaf spring to the inside of the other leaf spring straight across. This will give you the center point for the axle saddle mounts. Dont forget the u joint angle measure ment as well when you attach the new mounts.
why do you want to put a 8.8 in that thing all my buddies run mustangs ant those 8.8 rear ends are c clip axles those things snap on almost a daily basis go with an 8 3/4
or a dana 60 but if your just parcial to ford stuff sling a worthy 9 inch under it