I make a lot of pastrami, and for the most part, it sounds like your on the right track. I normally corn/cure my own briskets, but there'e nothing wrong with starting with a brisket already corned.
As far as suggestions are concerned, can soak for 12 hours, but you don't need to. A through rinse followed by a 3 hour soak will be OK. Change the water once.
Toss the packet that came with the briskets and prepare your rub. What you're considering is OK. My "Go To" pastrami rub follows, and you may want to consider it:
- 3 parts cracked coriander seeds
- 2 parts cracked peppercorns (mix of black, white, green, and pink)
- ½ part cracked brown or yellow mustard seeds
- ½ part minced or granulated garlic
- ¼ part cracked juniper berries
Following the rub, wrap the brisket in foil, or place in a large freezer bag. Put the briskets in a container and place heavy weights on top. I use 2-3 foil lined bricks. Refrigerate for 3 days. Then smoke to the temperature you mention. When finished, return the briskets to the refrigerator container, apply the weights, and hold it for 1 day.
The final step in making traditional pastrami that's often overlooked is steaming. Remove the briskets from the refrigerator, and put on a rack in a roasting pan that has around 1" of hot water in it. Cover the pan or wrap it tightly with foil and put it in an oven pre-heated to 250F. Take it to 165 IT then remove, wrap loosely with foil for a few minutes, then slice, serve, and enjoy.
