U-Haul still makes hitch assemblies that are built specifically for the 69-72 blazers. I haven't purchased one although I think a few others on the board have. Searched and found at least one post on it:
U-Haul part # = 75914
Link for pix and part number on an installed truck - start at post #30.
U-Haul details -
https://www.uhaul.com/Trailer-Hitche...eId=24161&ship
Install directions
link (from U-haul site)