If you are brining the 3.7 lb for pastrami, you'll find it on the small side for the amount of work, patience and amount of finished pastrami. My first pastrami was from a 4 lb brisket. We finished eating it in a few days. Just from picking at it, reubens and a pastrami hash, it was gone before we knew it.
The last pastrami, I cut from an 18 lb brisket. I cut the brisket in half and brined for 15 days. This was enough to eat, vac seal and freeze a large portion.