I've been in on cutting down several older GM Truck step side boxes.
DO YOURSELF A BIG FAVOR measure the length you need to have the box end up being total at either end. cut the end off, drill out the spot welds and move the stanchion back to the correct spot. The taking a slice out of the middle and welding two halves of a stepside back together is nonsense. No way around it on a fleet side but once you take the stanchion/stake pocket of it is all flat metal on everything except 54/ 55.1.
You have to go by the difference in wheel base not what some individual claims that will cause you to end up with a junk bed side.
Short wheelbase is 115 inches
Long wheelbase is 127 inches that comes out to 12 inches difference at the front of the box to still have the correct spacing unless you created your own wheelbase.
This page from the GM herritage center archives shows most all of the differences in length but you have to calculate the difference in bed length.
It came from here
https://www.gmheritagecenter.com/doc...olet-Truck.pdf
That you may have to get to the long way by going to
https://www.gmheritagecenter.com/index.html and clicking on archives and going step by step to the right year.
The amount you shorten the front is going to be as I said the difference in truck wheelbase. the guy who said 20 was probably going to cut at 20 cut 8 inches off the piece he cut out and weld it back together with one long weld that would along with warping the bed side call for hours of grinding and smoothing to have presentable. That is a lot of extra work to avoid cutting out some spot welds.
The diagram shows that the difference in total length between trucks without counting the bumper as
Short box = 188-1/2
Long box = 207-3/4
Subtract 12 from the 207-3/4 and you get 195-3/4 = shortened long bed with nothing cut off the back.on short bed chassis.
Subtract 188-1/2 from 195-3/4 and you get 7=1/2 that needs to be removed from the back.