To read the original thread and discussion on this topic go here- LINK
THERE IS NO NEED TO CUT FRAMES< NEVER!!!! Do I need to say that again? NEVER!!!!!!!
To install a 81-91 bumper on a 73-80 frame, all you have to do is make one VERY SIMPLE (2 min hacksaw) cut on the 81-91 center bumper bracket and the bumper is a bolt on! Yes BOLT ON!
All you have to do is cut the center bracket along the crease on a diagonal. On the end brace the center hole (in between the the 2 welded on nuts that are usually used) is the very same hole that the 73-80 end brace uses.
I really don't how to explain this so I take a photo in the morning to show were the cut is made.
But to say this a third time, THERE IS NO NEED TO CUT FRAMES< NEVER!!!!