Do you have copies of the old speed manuals? "How to Hop up Chevrolet & GMC 6-cylinder" by Huntington, and "Chevrolet/GMC Speed Manual" by Bill Fisher AKA California Bill are full of great ideas.
Nothing beats cubic inches, have you considered a 261? For best possible performance bolt it onto a 261 block. Seems like you only had a single 1BBL carb when I viewed your bolt. You can weld up the chamber area for increased compression. There are a lot of pistons to choose from as well. A supercharger would look good on your truck if you can find one.
