I might be over thinkin this, so feel free to ignore me. What's the old saying "be careful of free advice, you only get what you pay for". Anyhow, I needed a brain teaser this afternoon and I am truly sympathetic to using what you have on hand to work with.
I have about 50 square inches of connection to my firebox. I didn't do any profound preplanning or calcs, that's just what I wound up with. My math says two 6 inch pipes would give you 56.5 square inches of connection (you know pie-r-squared).
My tuning plate is 17.5 inches X (up to) 60 inches or 1050 square inches providing radiant heat (the way I did mine I can do less but not more). Two 6 inch pipes, assuming you only take credit for the top half of each pipe gives you 1140 square inches for a 60 inch length (circumference = pie X diameter X 1/2 cause I'm only taking credit for the top 1/2 of the pipe X 2 cause you have 2 pipes).
My smoker works for me, I get good even heat distribution, helped a lot by the radiant heat and I can cook anywhere from 200*-375*. Given the similarity of the numbers, I think two 6 inch pipes, something like 48 to 60 inches should be pretty equivalent. In your situation, I would try the pipes.
To Zin's point, I think he's talking about getting the right exhaust and intake balance. Let's assume you want to use some of the same 6 inch pipe for your exhaust, you will have about 28 square inches of exhaust. Some of us believe in controlling the fire at the intake, so you only need up to 28 square inches of intake to be able to get a clean burn and control nasty smoke. Zin can correct me if I've missed his point.
OK, now my head hurts
. I'm gonna make a wild turkey rocks & feed the cows while everyone else double checks my math and offers alternative suggestions. GOOD LUCK!