After comparing my '80 (with stock rockers & doors) to my '73 it looks like the 4x4 tube will work with stock doors. On my '80 its about 26" from the back edge of the trans tunnel cover plate to the inside edge of the closed door. The outboard edge of the rockerbox, and my tube, is just under 26" from the same point on my '73. I think you could just weld some sort of a nerf bar right to the 4x4 and call it good. I've jacked my truck up from my reinforced rocker-boxes w/o a problem. Ive also had a couple hard landings on them.
To give a finished look I put in one of those front section rubber floor mats. Then I used some plastic sill plates that install w/ 2 sided tape at the door openings.
The only issue you will run into is mounting the weather stripping at the bottom of the door. You MAY get away with welding a piece of sheet-metal on top of the 4x4 tube and installing the weather stripping just like stock. Then let it hit the door at a slightly higher point. I'm not sure that will work though, it might be too close of a fit. You may need to move the insulation in a little bit, out of line with the vertical mounts.
If you do go this way it will leave the bottom of your door exposed. I think if you are planning to wheel it with the doors on, some sort of a nerf/rub rail welded to the 4x4 tube would be a must.
If you go for it pleases post pics. PM me for help, I'll offer what I can. To start you'll need 8' of 4x4 tube, I used 1/8" wall. Its 47" for each side, 90* cut at the front, 45* the back. The inboard side needs a wedge cut at the front measuring about 1"x8" IIRC. Then take a sledge hammer and pound it back shut. This will let it clear the kick-up that is in the rocker-box for the body mount. AFTER test fitting, weld the seam back up.
I can pull my floor covering and take some close ups of the finished job if you need them for some reason. But, there is no way to get back inside. The tube is welded to the rocker box and the floor.
