Recipe Talk

Need help, please.

I want to make a new recipe for Rhode Island-style clam chowder and it calls for 1 1/2 quarts of chopped quahogs. How much will I need to purchase in pounds of the live, in-shell clams? I have no idea as I usually buy the canned or already-shucked when I make chowder. Also, do I need to steam open the live clams or do I need to open the raw clams (difficult), chop them, then add to the chowder at the end of cooking? The recipe I have is not at all clear about this.