A two-barrel carb on a two-barrel to one barrel adapter might COST you 15~20 HP; it certainly won't gain you anyyhing (other than looks).
I am unaware of a good two-barrel intake, but if one exists together with the headers, you might gain 8~10 percent on a stock engine.
15~20 HP would be doable if you also do a cam change and some head work.
Jon.
