A peck is 8 quarts (=2 gallons). I assume you bought them in their shells. If there's no volume measure on the bag, dump them in a mixing bowl and measure. If it's the size bag I see around here, you don't have enough.
A peck is a measure of volume, and a pound is a measure of weight. So there's no real answer to your question. Do you have about 2 gallons of mussels? That's a peck, give or take. But I think 2 lbs is likely quite a bit less than 2 gallons.