You need 47/53 splash aprons for the 53 or earler bed. The 54 splash aprons are 2 inches shorter
My 48 has been on a 54 frame since August 89. The primary difference between 54 and earler frames is that the hump over the rear axle is a lot flatter and won't let you lower the truck that much without a notch.
Note the height of the qpron on this yellow truck and then look at the one on Hogfarm's 54 and how much shorter it is to make up for the lower bedsides.
I should have moved those chairs to get a side shot that day.