I've given this a LOT of thought for my '62 Suburban, which also has a wood floor beginning behind the passenger seating foot well area.
I hadn't thought of using a cargo van floor--good idea. I'd thought about modern pick-up truck floors, but finding one with the wheel well notches in the right place would have been difficult.
After 50 years, the stock plywood floors are often rotted, or dry rotted. Also cracked. I haul a lot of stuff in mine, and have noticed a crack that opens up with weight.
My plan is to use a couple pieces of sheet metal, and a bead roller to roll wide reinforcement ribs into it, just like a truck floor. This will require making my own bead roller, since the throat will need to be half the width of the floor, but that's fine: I've got a Harbor Freight bead roller as a parts donor, and will always be able to use a deep throated bead roller for future projects.
Regarding the different thicknesses of the wood versus metal floor: Rubber spacers, isolator bushings, pads, etc. will cure that. I haven't started the hunt for that yet, but McMaster-Carr will be my first (and probably last) step in that search. That figure in at some point, but there are stringers/ribs going side-to-side under the floor. If you don't have those any more, get some reproductions for a wood bed pickup truck--they make all the metal. It's stamped U-channel, usually with a flange along both sides of the top. Cut to length, weld in place. I don't think you could do better than that. If you balk at buying repros, think of the cost and aggravation involved in cutting those out of a pickup truck or van at the junk yard.
I won't be able to find a sheet as wide as the truck floor, so I'll need to splice two together, butt-welded down the middle. Shouldn't be a problem. (hmmm... after writing that, it occurs to me that I do not, in fact, need a bead roller with that deep of a throat!)
I like the idea of a metal floor for several reasons: I'm not satisfied with the seat anchors, but more importantly, the seat belt anchors. I'd like something a little beefier. And, with a metal floor, I can use screws with winged tops going down into the floor, rather than fighting with the carriage head bolts coming in and going out of the key holes in the metal plates.
As someone else said, it also opens up the options for cargo anchors. And, it opens up the options for in-floor storage compartments. At this point, I'm also seriously considering using the spare tire well from a '55-'57 Chevy station wagon, which accesses the tire from inside the cargo area, rather than under it. It'll be a lot easier to change a tire (and check its pressure periodically) than when stored under the truck. The station wagon tire wells are available as reproduction. That would also let me use a repro wagon gas tank, and fuel sender. I'd rather have a repro gas tank than try and patch up my Suburban tank. The tank would probably be cheaper too.
As for sound, I'm going to spray the underside of the floor with Body Shutz, and then put cargo area carpet and jute backing down. If I get REALLY creative, I might make some metal strips to slide the cargo on, like the old Jeep Wagoneers did. For that matter, I've also thought about swapping in a Jeep Wagoneer rear seat: it folds forward/down to extend the cargo area. I've only looked at that once though, so it may or may not happen.
I drive mine daily, and just did a 900 mile round trip to a car show this weekend, where we camped in the back of the Suburban. More modern conveniences like interior access for the spare tire, fold down rear seat (rather than take it out at home) would benefit me a lot--I'd actually use those features.
-Brad
