Start with Prime or Choice brisket, flat only. I use Bon Apettit's recipe for
Corned Beef. I've found that a brine is preferable to a dry cure in final taste and texture. For my spice rub I use a mix of 4 parts black pepper, 2 parts coriander, 1 part mustard powder, & 1 part granulated garlic. No need for any additional salt. I cook mine at 225 on the BGE with a water pan on the plate setter. I wrap in foil when the meat hits 175-180 and pull it at 200. I prefer this method to that of genuinely steamed pastrami, but, you may want to try it both ways.
Last note is quickly vacuum seal and freeze any leftovers. For whatever reason it seems to go downhill quickly in the fridge.