I remember the "camper special" Ford's and Dodges of the 70s that had the boxes built into the lower parts of the truck bed on the passenger side ahead of the r r wheel.
I'm redoing an 85 d150 and was wondering if there's a toolbox made that's similar in size to those, that could be adapted, I know I would have to cut a hole in the bedside to be used as the "door".
Thoughts,ideas on how to to make it happen?
What was the back of the box anchored to? I'm sure the bedside wasn't meant to be counted on to support the weight especially the weight of what was loaded further back in the box?

As I remember, that box in the bed side was called a "cargo door" or an "Avenger Door" because it was an option under the Avenger truck package.

I've only see a couple trucks with them still intact, but I saw a lot of trucks with holes where they used to be. Seems they must not have held up very well. I don't believe they were very deep, maybe 6" front to back. I believe they sat just below the bed floor, and any support past the bed side likely came from the bed floor.

If I was going to do something like this, my biggest concern is keeping the stuff in the box dry in wet weather (and winter salt). Every time the truck is in reverse, that area the box is in is subject to whatever the rear tire throws at it, and everything below what ever inner fender, all the way to the ground, is present is game to end up in, at, or on that box tucked inside of the bed's outer sheet metal.

With that in mind, I would want the box attached to the truck frame (like running boards used to be attached), with a splash guard starting behind it (from the bed floor), down the entire rear side (between the box and the inner fender) ahead of the rear tire, and extended the entire length of the bottom of the box. By theory, the box could extend from the bed side all the way to the frame rail, and from the front of the bed to the rear inner fender in front of the tire, but the reality is the box size will be limited by the weight it will be hauling. I suspect the reasons the failure rate was so high was because the box was often overloaded.

Years ago I when I built my 50 Dodge 4x4 (a 50 cab on a 77 1/2 ton Dodge 4x4) I added a box above the running board on the passenger side, but under the cab. It was 6" high, 24" long, and sat on the angle iron framework the held the running board. Even with it being protected on all sides from any water splash, the inside of that sealed steel box was always wet, and quite useless. In the picture you can see the door for the box. When I redid the truck a few years later, I removed the box.

To get a flush mount door I'm sure the bedside was stamped that way and the box had to be gusseted with spot welds above the opening. Maybe that option had thicker gage steel.

Everything had thicker gauge steel back then. I think toilet paper is thicker than the sheet metal on my 12 Dodge 1/2 ton.

Some were surface mounted and some had a special stamping to flush mount the box. Appears to maybe be an 70s vs 80s thing? Box was made of fiberglass and supported by brackets that bolted through bed floor. 31" long, 15" deep, 9" tall.

I remember years ago back in the 90's. We were on a construction job site and I had my newer 96 Ram V10 when everyone would still like to check them out because they were still new in design and fresh. There were 2 white 8' box 2nd Gen Rams like mine there from another contacting company and both trucks had both bed sides that flipped up 180 degrees exposing a bunch of individual storage compartments and shelves for tools and supplies. When closed, they looked like a 100 percent stock pick up truck. They were cool. Only time I seen them and have never seen them again since.

Innovative Truck Storage Carson City,NV. Don't think they are still around.

I saw one of those on a dealer lot, used, in Saginaw Michigan around 2000. Glanced inside the bed and it was flush on the inside of the fender wells, so cool to haul 48" wide plywood. IIRC I opened the tailgate (which was stock) and there was a pushbutton release in the back that opened the side. Couldn't find any manufacturer tag on the box, dealer was closed, and when I got time to call them it was already sold. Don't know what the cost was, but unless it was crazy expensive they should have sold well if advertised correctly.

I remember seeing these back in the day. Almost pulled one out of a yard I came across in the 90s. I seem to recall there were a couple of brackets that were attached to a perimeter ring on the opening, straps that ran under the box, and bolted to the frame.

Biggest pain wouldn't be cutting the opening but reinforcing the cut out to not bend when being closed. The factory rolled the edges to give rigidity and there wouldn't be enough left after a cut to duplicate this. Of course, you could always cut an oversized piece from a scrap yard truck with enough extra metal to roll the edge and to make fit.

The originals weren't particularly weather proof, but you could remedy this by making a two step lip with gasket similar to what modern motorhomes use for their under chassis storage compartments.

Speaking of which, there is a company out, Lippert, there that makes auxiliary storage systems for RVs that mount in a frame that attached under the body and slide out for access. Not nearly as cool as an integrated box, but could be an option.