One approach would be to scale the recipe for 4 gallons (you can do that with a button in BeerSmith): Collect 5.5, boil down to 4.5. That might (barely) be enough headspace.
In order to do a proper 5 or 6 gallon all-grain batch you really need a 10 gallon pot. I once boiled a 5 gal extract batch in a 7.5 gallon pot, but it was touch and go, and that's not going to work when you start throwing in lots of flavor/aroma hops (and you will).
How about just skip the 5-6 gallon stage and jump directly to 10 gallons. I wasted a lot of money on Blichmann and Polarware pots to setup my 5 gal system, but it only cost $40.00-$45.00 each for the vessels in my Sanke 10 gal system. I jumped to 10 gal because it takes exactly the same amount of time whether you're making 5 gal, 10 gal, or 10 bbl (and then felt like a real dumbass for not doing that at first). If I could find a 30 gal stainless vessel for a reasonable price I'd jump to 20 gal right now.
Charlie