I have been wanting to do this for a long time but just got around to it.
As several others have done, I made some inserts out of 2"x4" rectangular tubing. These go in the end of the frame to strengthen it. The inserts are about 8" long and on one end you have to cut and re-weld it so it will match the countour of the bottom part of the frame.
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You also need to make some shims to take up the slack between the sidewalls of the frame and the vertical walls of the insert. I used 3/16"x2" flatbar, drilled a half inch hole, and then welded them to the outside walls of the inserts. These shims keep the sidewall of the frame from collapsing onto the insert when you tighten the hell out of the horizontal mounting bolt.
Then I made a stiffner bar out of 2"x2" heavy wall square tubing. This got welded between the inserts where the frame is notched for access to the original bumper mounting bolts. The inserts already eliminated access to these bolts so I decided that adding this stiffener couldn't hurt.
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Then I mounted some chrome tow hooks from mid 90s F250/F350.
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One thing I would do differently next time is to weld nuts on the inside floor of the insert so that you could just thread the mounting bolts in.
As it is now, you have to drop the rear mounting bolt for the tow hook down from inside and put the lockwasher and nut on from the bottom.
This is because you have to put the bumper on and drop a carraige bolt all the way down through the bumper, frame, insert..........and out through the bottom.....................which of course eliminates access to the bolt head so you can't tighten it. I got lucky and one is real tight and the other is kinda tight so I will have to keep an eye on it.
I am now ready to pull all my neighbors' 4 door Jeep Rubicons out of the snow banks this winter !
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1977 Sport, 351w OBDII EFI motor, 4R70W auto, 4:88 gears, ARB lockers, 3.5" suspension, 33" tires.