New member with a 1960

[The1960APE]

I do need some advice regarding the bed, in the mid section (Tire Area) of the bed it measures about 49-1/2 wide, at the front and rear of the bed it is right at 50 which leads me to the assumption that the center needs to go outwards, is this correct? I see no way to bring the front and back inwards, I was thinking of building a jig and using a bottle jack to push the center area outwards and brace it out until the bed floor is installed. What do you guys suggest?