Ported and polished intake manifold?
 

I have a 98 Gt convertible and I was thinking of getting my intake manifold ported and polished. Is this a good idea? How much horse power will it give me? any suggestions?

Naw, your intake sucks...you'll need to get the PI heads/intake/cams from a 99-02 GT if you ever wanna get out of the 14's NA with your car. Even if you go the blower route, your stock heads will restrict the power greatly...