Build suggestions for 340?

For a streeter,
IMO;
Since it is going into in a lightweight-A, with log-exhaust, and 3.23 gears;I would treat it as a big-bore 318..
With 3.23' and a 4-speed, first gear will be a tireburner no matter what cam you put into it. That means Second gear is gonna be where all the action is.
With 25" tires, 3000rpm is 36mph and 6000 is 72mph
You need to decide where you want the bulk of your power to be, then chose a cam to put it there, and finally, adjust your cylinder pressure, with static-compression ratio, to make best use of the chosen cam.
The iron open-chamber heads will limit your cylinder pressure.
400hp requires a bigger cam, which drives the operating rpm up.
Just remember, that 400hp is not likely to arrive until close to 6000 rpm/72mph in second gear, and 400hp will spin any street tires you can fit in the factory tubs, all the way thru second gear. Is that what you want?
I'd settle for 380/400ftlbs at 36 mph, and all-done by 60mph/5000 rpm with 3.23s..........but that might just be me,lol.

Oh sorry, the "before pic" shows headers, that will not change my discourse