I am going to build a 383 and plan on using my stock looking TPI unit. What I mean is that I am going to keep it looking like a traditional TPI unit. I will upgrade to larger runners and a larger base unit, but the plenum will still be there. How much power can I wring out of this with the plenum restriction?

The plenum is not a restriction. The power restriction will be the diameter of the runners, and the length of the runners forcing a low rpm power band. Your best choice for a base is the Edelbrock (just because it's the cheapest $, they all perform about the same) and either SLP or AS&M semi-siamesed runners.

You should be able to get 350+ rwhp pretty easily with a 383 I would think.

Good luck.