Unless you have the budget to also upgrade the trans (or can keep your foot out of it) you may want to reconsider at the moment. Cleaning the engine compartment like grizwald said will probably be a good starter. I'm guessing you don't have emissions there... if you want to build the 4.3 you can go with a carbed setup to keep it somewhat simple, but it still has the capability of making a decent amount of power (around 300hp based on the build below).
http://www.hotrod.com/techarticles/engine/113_0208_chevy_43l_262ci_v6_engine_build/index.html
http://www.hotrod.com/techarticles/engine/113_0208_chevy_43l_262ci_v6_engine_build/index.html