1st of all, welcome! 2nd of all, better off not worrying too much about horsepower. Best gains is what you will feel by the seat of your pants. Any performance gains going forward will appreciate dual exhaust. I personally have a Weiand action plus on a 318 right now and like it. The 318 I have is 100 % bone stock except the carb and intake. I will lay rubber all over the pavement and I shift it about 4800 rpm's, and it pulls well all the way there. Biggest thing for you might be your heads. Your heads do not have hardened seats, and if it had unleaded ran through it for thousands of miles over the years, then you may have a bunch of leaky valves and that hurts performance. A 340 cam size works well too. Time it, tune it, and tweak it.