What exhaust sounds good for 340s?

The muffler is the big dictator of sound while the rest of the components shout into the loud speaker (AKA muffler)
There are basically 3 types of mufflers.

Absorbing like a Borla or Hooker/Dyno Max straight through muffler with a absorbing material like stainless steel wool or fiberglass like a glass pack.

Chambered mufflers like the Hooker Aero Chamber or a Flow Master.

Turbo like Thrush mufflers & Dyno Max and many others.

Your best bet is to attend a few car meets and shows to listen for the sound you like and ask what muffler is being run and then ask about the engine build starting with cam size, compression and what headers they have or exhaust manifolds they run. Because those are also dictating sound.