The TH350 would probably be the cheapest mod to add some acceleration to your Chevy II. You should be able to find a decent used one with converter for well under $200. Unless you want to step up to anoverdrive transmission, go for a higher rear end than the 4.11s. Something like a 3.55 should be a good compromise; you probably have a 2.73 or 3.08 now.
For a better carb, you will most likely need to go with a different intake, such as an Offy and with your stock motor, the Weber two barrel would work well. Might as well go with headers at the same time. Sometimes you can find these pieces on ebay fairly cheap. Good luck with the 68.
