I would recommend getting a bigger pot. Wally world has imusa aluminum pots for cheap.
If that isnt an option then brew the beer twice. Split the recipe in half and brew them as 2 beers at 2.5gal. Once the beers are done mix them in the carboy.
Secondary2: Skeeter Pee
On Deck: DFH 120min IPA, Arrogant bastard clone, Etc.