how much hp will i get from doing a port and polishing on my intake manifold & put on a 58mm Tb?

 

I assume you are talking about opening up the intake to match the 58mm TB. In stalling a 58mm TB on a stock engine will get you next to no gain. The 48mm TB is perfectly sized for the LT1 and flows more than enough.

What future mods are you planning. Most any Heads/cam car between 350 and 400RWHP only needs a 52mm TB, it will flow 750 CFM and that is plenty for an engine in that range. I personally know someone running 11.60 with a stock TB.

58mm is over kill unless you are blown or have a huge cam.