That's entirely reasonable and quite possible. Start from the start: What is the year and model of the vehicle you're working on? How many miles are on it, has it got the original engine, what carburetor is presently on it, and what kind of emission tests (if any) do you have to pass?
More specific advice once you've told us what exactly you're starting with, but no, a 2½" exhaust is not a wise choice, no, headers are not necessarily a wise pick, and no, a 2-onto-1 carburetor adaptor is not better than a stock 1bbl (and there's no reason to use those dumb adaptors; a 2bbl intake for a slant-6 is easy to buy or make).
