Your conundrum is much simpler to address than if you were trying to put a '54 bed on an earlier frame. The '54 bed is deeper than the earlier ones. The '54 frame is lower to allow the deeper '54 bed to fit. Seems to me that you just need to use taller blocks between the frame and the cross sills. Your '47-'53 splash pans should fit if you jack the bed up to the proper height.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission