Reading the engine description, the first thing that jumped out at me was the single plane intake. A single plane intake is a good choice if you have a dedicated race engine that lives at high rpm, or a modified, big-cubic-inch engine.
305's are definitely not a good choice for any kind of serious, high performance and do not need a single plane intake.
Switch out that single plane manifold and install a basic dual plane manifold. Don't buy a new one. Check FB, C-list, and your local Chevy friends for a used one. They do not wear out. Just check the threads to make sure some monkey didn't strip them out tightening some accesory bolt. Don't bother with a high-rise dual plane or an air gap dual plane. You just don't need it. Most folks like aluminum intakes, but an OEM cast iron one will work just fine for your application.
By going from the single plane to dual plane, it should bring down your rpms, make your engine run smoother, and basically fix your problems.