Help an old man tune a new to me 340

Initial timing is at idle.
As you increase rpm, the mechanical advance inside the distributor increases timing. You limit the amount of mechanical advance by stopping the weights from moving further out. You do that either by welding up the outside of the slots or by installing a limiting plate from FBO. The plate "limits" how far out the weights can move. You want total timing (inital plus mechanical) to be 36ish or less.

I use an FBO plate and it works great for me.