Also wanted to add, if you cut it into chunks you have more surface area for more "bark" which can enhance the pastrami if you like to heavily pepper pastrami.
SO: I put my serving sized pastrami portions into el-cheapo sandwich bags. Then, those bags are put in ZipLock Freezer bags and labled.
Same as I do my pre-cooked bacon.
I just pull out one little baggie for a couple of sandwiches as needed.
Never any problem with it lasting a long time without freezer burn that way.