Does anyone know if a push bar from a 1973 F250 will fit on a 1988 F150?
It mounts to the frame and the bumper.

 

Not easily. The bumper from a 73 is a lot shorter and has a sharper angle. The 88 bumper is almost flat on the front. It could possibly be made to work with some fabbing though. I used a bolton pushbar for an 80-86 truck on a 79 F-150. I had to drill 4 holes in the bumper and made some aluminum spacers to fill the gaps on the bottom mounting parts.

Here's the pushbar on the 79.

Here's the same pushbar on an 85 F-250.