I would put the ARB and 60 series knuckles on the 40.
The way I would do it is to pull the Pig axle apart and pull the diff out. Pull the 40 axle apart and pull that diff out. Swap diffs. Put Pig back together. Then do the 60 series parts swap on the 40 as you reassemble the axle.
Leave the '74 axle out of the picture and leave the questionable 3rd out of the picture. Unless you want to go through the fun exercise of rebuilding the 3rd beforehand, so you know it's good. Then you could swap that into the Pig and maybe not have too many irons in the fire at once. I would still leave the spare axle housing out of the picture though.