Can you post the brine recipe? I'm assuming you are going to cure this using a wet brine?
I should be able to covert it to cure #1, and adjust the curing times, for you to use in the future. Saltpetre is not a dependable curing agent. I know people have been using it for ages, but since cure #1 with sodium nitrite has been on the market, that is more reliable, consistent, and cures much faster than saltpetre.
If you want to go without the saltpeter or cure #1, and if it is a wet brine, you can use a 10% salt solution brine, and that concentration will provide enough salt to retard any harmful microbial growth while smoking/cooking in temperatures in the hot smoke range. A 10% brine is about 1 pound of salt per 1 gallon of liquid. When I make pastrami, I mainly use the cure for color and flavor, since I smoke/cook my pastrami above 200°F. So if you smoke/cook at 200°F or higher, you can use the brine recipe you have without saltpetre. Just note, if you use salt only you cooked pastrami will have a grayish color to it, like you would expect to see in a cooked pot roast, and lack the flavor associated with sodium nitrite.
If your roast is a very thick cut, you may want to inject, or if you don't have an injector you may want to slice it so it is less then 4" thick.
One other thing, sea salts contain many minerals that may have an effect on your finished product. It is better to avoid sea salts, and to stick with non-iodized table salt, canning salt, or Kosher salts.