Highest LIft cam with 68 340 with stock intake and carb

I'd say headers 1st on a cam with decent amount overlap needs a good flowing exhaust over intake carb. If exhaust is free to leave your engine it will help pull air in even throught a restrictive carb intake. But with poor exhaust hard to get good air flow if the exhaust is trying to get out the way it came in (intake). Seems to me your engine needs headers, airgap and 750 Holley or smaller cam.