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.

That's why headers was first. It will take a custom cam to do much with stock exhaust manifolds, and you'll still be leaving power on the table. A friend's Stock Eliminator 351C ford mde 485hp with unported heads, stock 9:1 static, stock iron intake, and stock autolite carb. But it had big dollar stepped equal-length headers and a custom hydraulic cam with stock lift to work with it all.