How I built my Dual Snorkel Intake

I used this article at ThirdGen.Org by Kevin Vandevenne as a reference.

Step one:  Find a second air cleaner with the large style intake horn.  I got mine for $8 at a local junkyard. I pulled it out of an early 80s or late 70s Chevy full size pickup. You'll do best if you get there early, as this seems to be one of the first items that gets pulled off and lost when people grab stuff off of the engines. If you have one of the restrictive intakes on there now, you should get two of the large intake air cleaners, one to cut up and one to use.  Some may want to get two anyway, so they can retain the stock one in case they mess up.  That  is the cowards way.
 Both 2-barrel and 4-barrel size air cleaners are available with the large intakes.  As far as I can tell, the large intake styles are the only ones of the 77-90 b-bodies to have a provision for attaching cold air intake tubing.  Here are some pics of what I'm talking about:

Small Intake:                Large Intake:  See the little nubs at the very end?  Those hold on the cold air tubing.

Step Two: Start cutting (use goggles!)  I used a dremel and about 10 cut off disks to remove the intake from the donor air cleaner.  Notice that I left about 1/2 inch of the bottom of the air cleaner on, as well as an inch on either side.  That is so that I could have plenty of mounting points.  Around the top edge of the air-cleaner there is a ridge of folded metal that serves to help seal the lid to the air cleaner.  You want to cut this off of your donor intake. You want to cut it from the inside of the box, about half way down the total length of the folded part.  Only cut through one layer of the metal.  I cut it from the outside and removed the whole thing.  If you do it the right way, you can have the top of the donor piece butt up to the bottom of the folded ridge on the air cleaner when you attach it.  Cut off the thermac valve.  I also cut off the piece on the underside for the hot air intake hose.  This is your call, it doesn't really matter, it may be easier to plug with it on. See my intake here:

Step Three: Cut a hole in the air cleaner for the new intake.  First hold the donor intake up to the air cleaner where you're going to want it to go.  Clamp it down.  Next use spray some spray paint down the donor intake.  This will mark exactly how big & where the hole should be.  I used the dremel to cut out most of it, then a grinding tool to get the curves just right.

 it is with the paint:         here it is cut out: 

Step Four:  Cover the holes on the donor intake from the thermac unit and hot air inlet. I cut out some 18ga metal studs with tin snips.  It works fine, but doesn't look great. I wanted to rivet everything, but didn't have a rivet gun, so I used some tap-it metal screws.  These should work fine, but they don't look as clean as rivets would have, and the screws protrude into the intake air cleaner assembly, although it's not really a problem (I cut off the sharp tips with the dremel just to be sure.

Step Five:  Sand and paint to taste.  This is easier to do when it's still in pieces, as the final product is a little awkward to work on.

Step Six:   Clamp the donor intake to the air cleaner with the holes lined up.  Again, I screwed it down, you should probably rivet it.  Here's a pic of the test fit before I screwed it down or painted it.

Final Step:  Install the unit in the car.  The intake on the left is the stock intake, and the cold air tubing on it is stock.  If your car doesn't have it (it seems almost to be a random option) you should be able to find it in the junkyard pretty easily.  The intake on the right is the one that was attached.  The tubing on it is stock from a Chevy truck.  On the truck, it plugged into the fender.  I've just got it wedged down behind the lights.  I will try to get the stock Caprice tubing pieces on my next junkyard trip and reverse it to give the intake a more symmetrical and neater look.  All in all, I didn't notice much of a performance gain (it's a 305).  It did growl a little more.  One interesting note, it seems as if some B-bodies came with the hot air return on the driver's side, some on the passenger side.  If you got one of the manifold shrouds from each, you could hook up hot air return on both snorkels and look really stock and slick, however then you would have to plumb another vacuum line to the second thermac.

Latest Update: Found some duct pieces that make the driver's side look a lot slicker. The scoop is off of a third gen Camaro, but I don't know what the rubber elbow is off of.  Maybe an Astro van.