Start by getting the book Chevrolet Inline Six-Cylinder Power Manual by Leo Santucci. Most any on-line bookstore has it. There's also a lot of good info at
www.inliners.org .
First on my list would be to make it breath better. Stock is under carbed and the exhaust is restricted. Dual carbs and a split exhaust will be a big help. Also an electronic ignition, if you don't have it already. After that you get into cams and head work.