340 deck clearance

Depending on the model year, the 340 did have pop up pistons but they were changed to flat top pistons in later model motors. Personally I don't really like that so I go flat top pistons with closed chamber heads.
Now to answer your question about the Tdc with pop up, I don't know.
If your motor has pop up pistons you'll need an open chamber head (closed chamber would have the piston hitting the head). With a flat top piston you can use either the open or close chamber head.