if you are going to do a full boil, you just need to increase the amount of wort you boil to start with, there's no real adjustments you need to make. stick with the 2 gallons for steeping, then you want to somehow measure what you get from that (the grains will absorb some of the water) and figure out how much water you want to start with so you end up with ~5 gallons after the boil. there is no exact formula for that, but depending on the type of burner you use (stove, turkey fryer, etc.), your boiloff rate should be 1-1.5 gallons per hour or so. hop utilization will be different, but i wouldn't worry too much about that.