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I've got to do a tail gate post replacement sometime soon on my '74 and was wondering the same thing. I haven't taken a good look at it yet but if I tackle it soon I'll get some pics for you.
Not a replacement, but back in the '80s I took a 1/4" thick, 3 or 4" wide piece of steel and bolted it across the rear just under the rounded lip at the rerar of the bed, length from side to side. Under the bed, I added some 1/4" steel brackets from frame to the back side of this piece.
Then, on each side, I took and notched two pieces of 3/16" thick, 1-3/4 X 1-3/4 angle, notched so that I could bolt it to the 1/4" piece I described above, and the short side extended into the bed. I bolted it to the factorty corners with several bolts from both sides.
Down at the lower end, near bed floor level, I added some extra gussets with a 45 degree angle.
To this day, I can drop the tail gate and you can grab a bed side and shake as hard as you want / can and they do not move.