Those box conversions from acdelete.com are an insane amount of fabrication work. Totally not worth it.
I've done the factory non a/c swap on both of my Oldsmobiles, and it's really simple.
I copied and pasted a how-to write up I did a while ago for you... Hope this helps.
First off, go ahead and remove your entire dash. You will need it out to get the a/c ducts out, and the non a/c ducts in, they are different. There is a brace you will need to remove that hangs from the middle of the inner firewall that obstructs the ducts that is held in with two 13mm bolts. Remove the a/c heater box and all the related components. You might have to drop the passenger side inner wheel house to get to some of the lower bolts. Mark the connection where the fan switch plugs in, as the non a/c fan wires will plug into this same spot without any modification.
Install the non a/c box and fasten it to the firewall, and use a lot of strip-calk or your preferred product to help reseal it. Once that is in, you can go ahead and install the air ducts and replace the dash brace you had to remove to get the old ones out. It's best to leave the vent control cables that go under the steering column attached to the box, but if you (or somebody else) removed them, put them on now and let them dangle where they want. Now you can start to re-install the dash board, being careful not to pinch any wiring and making sure that the cables for the vents operate smoothly and do not bind. When you have everything where you want it, tighten all your fasteners, and reconnect your wires. The wires that hook into the original plug in that you marked go out through the corner of the heater box and attach to the plug-ins on the front of the heater box. The glove box liner is different and you'll have to either swap a correct one, or cut down your original one to fit. I've done both and it's a lot easier to find a correct one, although removing and reinstalling the lock cylinder can be pretty tricky. Hopefully you can find a set-up out of another Chevy, so that the bezel under the column that holds the vent controls will fit properly, since all the different makes have different style pieces.
The best recommendation I can make is to either find a car that still has all the pieces intact, so you can make sure you get everything, or find one that is being sold where the seller has been careful and has included everything necessary to complete the swap. Buying an incomplete set up will lead to some major headaches.
If you're going to snag one out of the bone yard, make sure you get the entire heater box assembly, the screen on top of the cowl, the ducts, all the cables and wires, heater controls, the glovebox liner, and the vent block offs for the driver side vent on the instrument cluster, and the pass side block off on the far right. The more intact you can leave things, the better. Look on eBay, as there are usually a set up or two on there that somebody has taken out for sale. Look at all their pictures so you know what to keep an eye out for.
I hope that helps. I think I did a pretty good job covering what you'll need to know, but let me know if you have any more questions.
