Old vans from the early 60's, such as my 1965 Ford Econoline, are designed to be compact and are considered the first minivans. There's a good amount of room inside but no place to lock up valuables out of sight unless you install a locked box somewhere. If you don't mind doing some cutting into the floor, and permanently modifying your van, then there are two locations where you can utilize unused space underneath the van. I call it a smugglers box because it's more fun than a battery box or extra underfloor storage and reminds me of smuggling Droids and Jedi Knights in a film made 11 years after my van was built. Lets get started...

To do this you will need a lot of gear including welding, cutting and grinding tools. Because I live in an apartment and don't own any of these tools I made it at Tech Shop. Check to see if you have a Tech Shop near where you live.

http://www.techshop.ws/

For my box I used approximately:
8 feet of 1/8 x 1 inch angle iron
8 feet of 1/8 x 2 inch mild steel bar
15 feet x 1 ft 16 gauge mild steel sheet metal
2 cans of Rust Oleum primer and paint.

Step 1: Locate the Spot You Want to Place the Box, Measure and Cut.

In an Econoline there are 2 natural spots to add a box. The one I am using here is just inside the side doors behind the rocker panel. The box I built is 32 in long x 11 in deep x 8 inches wide. I used these dimensions to make it possible to hold up to 2 batteries. Your dimensions may vary or need adjustment as you progress depending on your needs and also how accurately you are able to cut the hole into the floor.

To cut the hole I identified natural lines that will not intersect with the frame or other important structures underneath the van and also worked with the ridges that are embossed into the sheetmetal floor of the van. I cut the floor with a portable Plasma cutter which goes through metal like warm butter but you could also use a high speed cutting wheel.

Measure out the hole you will cut and draw a line to follow with chalk or grease pencil. It is a good idea to drain and remove your gas tank before you do this step.

Step 2: Square Up the Frame

Once you've cut the hole it's time to grind down any rough spots and even out the lines so they are nice and straight. The edge doesn't need to be perfect because it will be covered by the angle iron but it should be fairly straigh so the lip of metal gives something to hold on to. Remeasure the hole to confirm the final size. Cut your 1/8 x 1 inch angle iron to fit the length of the hole plus 1 extra inch on either end. you will notch a square out of the underside of the angle iron so it will have a lip extending out onto the floor. This will mate with the connecting bars that fit between. Line up the 4 angle iron parts and confirm that they are the correct lengths and that they mate well inside the hole that you cut. The function of this part is to grip the edges of the hole and be the main support of the contents of the box.

Step 3: Tack Weld the Frame

Use any extra angle iron and some clamps to make the frame square and flat and then use your welder to tack weld the corners together till it is strong enough to lift out and keep it's shape. For safety I usually remove my gas tank when I am welding on the van itself. If you would rather not do that you can try to clamp the parts together securely and weld outside of the van. Wear protective gear when you weld.

Once you have the frame tacked together you can begin to weld each joint where it makes contact with the other piece.

When you're done, grind down flat any rough spots or slag.

Step 4: Measure, Cut and Bend the Box Parts: Straps

I used a variety of scrap mild steel to build the box, some steel yards sell their leftover cuts by the pound so you can get a good deal.
for the straps that provide support I used 1/8 x 2 inch steel. The box is 10.5 in deep and 8 in wide so I cut each strap on the horizontal bandsaw to 29 in leaving about 1/4 inch extra for the bends. This box will be pretty rough so don't expect perfection and sometimes parts will not turn out. Buy a bit more steel than you think you will need.
After I cut the straps I measured where I wanted the bends and then used the steel press to bend 90 degree angles.

Step 5: Weld the Straps Into Place.

Once you have the straps cut you can place them onto the frame and check everything out. make sure it all line up and make any corrections that are necessary. If it all fits tight enough you can weld together as is or use clamps to hold the parts in place. I placed the straps so they are outside the frame gripping the edge and welded anywhere there was an exposed seam or contact point. My welding skills are pretty low but everything seems to have melted together properly. A made a lot of redundant welds to ensure it will stay together.
Once the straps are welded on it might be good to double check that everything looks and fits right by dropping it back into the hole in the van floor.

Step 6: Cut the Side Panels

Measure and cut the steel panels that will line the box. I used 16 gauge mild steel. If you would like to seal the corners really well, better than just a welded butt seam, it might be good to allow some over lapping flaps and then measure and bend the corners to conform to the inside dimension of your box. Test out the pieces for fit. You will most likely need to clamp the parts into place to get all the surfaces to form a solid contact for welding.

Step 7: Weld All the Side Panels

First clamp and tack weld each piece into place so that you can remove the clamps. Then you can weld each seem that you can get at until you are satisfied the box is solid. Grind off any excess slag.

Step 8: Primer and Paint

I rattlecan primered the raw steel with white since that was what I had in my paint box, then I applied matte black. I originally intended to powdercoat everything but that would require quite a bit of sandblasting. Rust Oleum is pretty durable and forgiving of imperfect prep. I may also seal the box later with some type of rubberized coating or perhaps a thick enamel paint. Let it dry. Drop it in and test the fit and finish. If you made the depth between 10.5 and 11 inches it should just barely be visible below the rocker panel if you squat down to look at it. At that depth if should be able to hold a standard car battery.

Congrats you just made a Smugglers Box for your van. Next I will add a lid but that solution may vary depending on what type of flooring, if any , you will have on top of it.