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I know you said frame mount was preferred, my Father in law built a pair for his '77 F-250, but was in the early '80s, I don't have pics. He used some open expanded metal mesh framed in about 1" x 1" angle steel frame for the platform, and some heavier about 1-1/2" x1-1/2" or 2" x 2" for support back to the frame. Pretty stout and they eased entry for him and my Mother in law. He had a fondness for getting firewood and come a day he got to close to a stump on the driver side, he bent it. He removed that one, and shortened the passenger side one then. After he passed away, his son removed the passenger side one too. I don't have any pics of his.
I used to have DeeZee aluminum extruded running boards, but I had steel hidden support. Very stout, but limited on how much foot could get on them in use. This time I found some CARR Super Hoops out of an estate sale, never used, I made my own hangers for vertical loading and brace back to the floor over the frame to support them in place, this way there is no twisting force on the cab pinch weld. 20" long, 16" of it is step. I know, not what you want, Wife loves them with pillar mounted hand grabs.
That brace is 1-1/2 x 3/16" steel, it'll collapse if hit. Then the step will pivot inwards. 1/4" bolts through alloy into brackets should pop then.
JJdel
There is a large thread on here about truck steps.
I posted my steps on there and they are bolted to the frame.
Someone here can find that thread. LOTS of good ideas on that thread..
Alex
Do you have access to a welder and can you fabricate? Or do you need bolt and go? No one really make a set of nerf bar tube steps that are dedicated for a 73-79, you have to get a universal set and figure it out. Or get these: