Dual Air Intake Setup

I have been passively working on a dual air intake for my suburban. I have a stock 89 GMC V1500 4x4 with a 350, 700r4 and 10 bolts front and rear. The programs that I have been using to read the computer are from here and I am (not bragging) getting proficient at location the programs to open TP5. It normally takes me a few times to get the programs in the right locations before I can log. Anyways, I recently took a trip to the cabin, 110 miles round trip with 4 lane highway and 2 lane back roads. I filled up before we left, with the trailer in tow, I got 20 miles per gallon. So I thought, is it possible to tune the truck to the point where I can get 20 mpg or close to it. I have 33" (31.9") tires on it so the mph is off, but the gps is just as or even more accurate. I put approx. 10 gallons of fuel into the truck a week and run 19.8 miles one way to work. Barring any trips off of course (which normally happens once a week) the doctor, license branch, store, guns shop, etc, etc), I'm pretty good at stopping at one station and getting fuel, but I can't get it filled up I have a hole in the fill tube and am waiting on the body lift equipment before I replace the fill tube, since it is rubbing on the frame rail and is the cause of the leak.

The last time I logged, after I got everything under the hood taped up (for testing purposes before I fabricate the real tubing that will be mounted and stationary). I went to the local pull a part and got every air intake tube that fit my test part that I could find. $4.63 and 3 hours I left with 42 air tubes I headed home to figure out the routing. Got all of the figured out, and tigged together the air cleaner housing. Installed everything under the hood and logging I went. I ran it for 45 minutes in the drive way, 60 degrees out side, had to kick the efan on (the little one on low), never went to closed loop once I was out on the main road (I live in a subdivision). Ran the back roads out to I-70 then up on the 4 lane hardpack for 8 miles, got off at post road and ran 21st street back to my subdivision.

For all of the run I am still running the stock setup chip, for a bunch of different reasons, mostly just playing it safe. Basically I want to understand the factory setup before I change a bunch of stuff and really don't understand anything! But I do have a few questions, Why would it not go to closed loop sitting in the drive way? 2. What is a safe blm number, I was up on the hardpack running 65ish and I remember looking over and seeing a blm of 131? 3. How far on the bom numbers should I go before backing off of my modding the setup?

Pics of the setup, I have decided that I will go with a 400 instead of an ls motor when I do the swap, so I really want to figure this out. I want to get the equipment set up and viable before I do the motor, that way all I have to is install all of the parts then just drive!!!