Only way I've seen 350's get over 15 mpg consistently is with EFI. Went on a 700 mile day trip in a burb with EFI, most of the driving was around 80 mph, and got 17 mpg. One low-dollar improvment is to change your ignition advance curve. You need a timing tape for the balancer, spring & weight kit for the distributor, and an advance timing light. Ideally you will be running 32-36* advance at highway speeds, firing well before the intake valves close. Should get you another 10-20% mpg on the highway. Went from 11 to 14 in my 69, 3.07 gears and th400.
Everything you mentioned will help. I have a 650 Holley 4 bbl on mine, but I got better hiway miles with the Q-jet. Problem was that the q-jet was worn out, and I couldn't afford a new one at the time. The mechanical lockout of the secondary does help a lot on the highway. Edelbrock is currently making new q-jets, but they're over $400.
