Thinking of swapping mufflers but got a question pertaining to my 340

On a daily driver it's really all about what sounds the best to you. The back pressure myth does have some truth to it, especially with modern cars, but not really how most folks think. In order to meet sound and emmission standards, modern cars run very restrictive mufflers and catalytic converters. The exhaust system design and engine tuning is set up to run the *best (hp, tq, mpg, whatever manufacturers actual goals are) with these restrictions in mind. The R&D runs very deep and inludes pulse tuning etc.
In theory slapping on a higher flowing muffler "should" make more power, but not for every situation, just like tuning to a specific AFR won't always make the most power either.

The only way you would know for SURE, was to run the car on a dyno back to back with your changes. Everything else is pure guessing.

But like I said, none of that matters if every single time you drive your car, you are thinking about how it would sounds, and not trying to shave tenths of seconds. Go for it, you won't hurt the engine or your street performance. The car will still pull hard and if it sounds better to you, your smile will be even wider..