You can put gears in now and posi later.
You didn't say how "high" you were going in ratio. You might currently have a series 2 or series 3 carrier. Depending on how high you want to go in ratio #, you might need to use "thick" gears to make up the difference or get a different carrier. (or use gear spacers- but I would advise against them in with any hopped up engine.)
What I'm trying to say is if you want to use the same gears with a posi center section, the current differential and posi differential need to be the same series. Just something to keep in mind.
Swapping from open diff to posi diff is relatively easy, keep track of the shims- left and right side- and install in the same position and the gears will be set up.