I will use flat top pistons not domed ones, so I can use a closed chambered head??? It is standard deck height right now but hasn't been checked and surfaced yet.
Even the flat tops will be above the deck on an early 340. My KB243's are flat top pistons, they're still .018" above the deck.
Closed chamber heads would have to be checked for clearance. Edelbrock's aluminum heads are closed chambered, and they make a special version just for the 340 that's basically an open chambered head to clear, so Edelbrock doesn't think a closed chamber head will work. I've heard it's possible, but you would have to determine exactly where your pistons are in relation to the deck on your engine. And then you'd have to check the piston to head and piston to valve clearance.
The other thing to consider is the compression ratio. If your flat tops end up .018" proud of the deck like mine, you won't want a close chambered head if you want to run pump gas anyway. With 65cc chambers I'm at 9.8:1, and it only runs on 91 octane with the total timing pulled back to 35*. Granted, that's on California grade 91 octane and fairly close to sea level, so your results may vary.