you should also check your frame for straight, square and sag as well. especially since it has possibly had a hard life at some point, noted by the stress cracking at the rivets etc. it's pretty easy to do and should really be done anyway even if not boxing the frame.
do a corner to corner check ensuring to hold the tape measure the same on each side and also pulling it nice and tight each time. I used a bolt in a hole that is in the same spot on each side so the helper could hold the tape without eyeballing anything. since it isn't really an actual dimension you are trying for but rather a difference of dimension you are checking a piece of wire with a loop on each end could also be used if doing the job by yourself. just pull it hard enough to stretch out the loop in the end of the wire so you get accurate results.
for the straightness check you can place a piece of masking tape in the middle of each cross member and then mark it with a sharp pencil at the midpoint, side to side, of each part. then run a laser or string line from front to rear down the middle. all the pencil marks should line up.
for sag you can run a laser of string line from front to rear down each frame rail. space the string or laser off the frame with a small length of wood or some other equal sized block, then use another block the same size to check the space between the string or laser all the way from front to rear on each rail. it should be the same all the way except for the obvious tapered areas.
these links may be useful
https://www.trifive.com/55-59Assy.pdf
this one has a few pages missing here or there but these trucks are basically all the same with the exception of the 58-59 frame length being different ahead of the front springs and few other differences between those years and the earlier 55-57's
http://chevy.oldcarmanualproject.com...esto/index.htm
it's good to download a copy of the assembly manual for reference in case the link goes bad. same for info from the other site
keep posting pics, we all love pics and sometimes gets us off the couch and interested in our own project again. sometimes you just hit a wall, you know?
here is a bit of info on fishplating a frame as well. lots of theories on whether the ends of the fishplate should be welded all the way around or if the weld bead should go off the corner and continue on for a short length of weld at a different angle then the fish plate in order to stop a stress crack
https://forum.millerwelds.com/forum/...sh-plate-welds
https://forum.millerwelds.com/filedata/fetch?id=507884
https://mechanics.stackexchange.com/...lating-a-frame
https://67-72chevytrucks.com/vboard/...d.php?t=123624
https://mechanics.stackexchange.com/...t-fish-plating
sorry man, I got on a roll when my coffee cup wasn't empty. haha. it's done now.
hope it helps somebody