Please forgive me, I know this reply is over two months past the original post but, Im not sure the math in the first reply is entirely accurate. A 48gal boil for a 31gal (1BBL) finished batch is appropriate. A 5 gal finished batch will have a boil size between 6 and 8.5 gallons, not pounds. Multiply that by 6 to reach 1BBL and your boil is between 36 and 49.2gal. The low end making some fairly weak tea based on todays typical Microbrews. One gallon is 8lbs so your weight is 288lbs to nearly 400lbs, depending on the size of your boil.
A high gravity beer is going to have a larger boil, towards (or beyond) 48 gallons, which will be the max for your 55gal brew pot. That will fill your pot to less than 4 inches from the top. I dont know how your boils go but 4 inches isnt a heck of a lot of room to me. If you look at Blichmanns own sizing guide youll see that 55gal is small for a 31gal batch. The other pots are twice the size of the recommended maximum finished batch except for the 15gal pot which has a max finished batch of 8gal. If you dont already have your 55gal boiling pot you might concider a larger one. Extrapolating from Blichmanns own numbers for smaller pots/batches, a 60 or 62 gallon pot seems more appropriate for a 1BBL finished batch.