Should I swap the 3.91's for 3.55 or 3.23

All right, an opinion question. You will get all sorts of varying responses to this question. Here's mine; a 340 (or any Mopar small block), a 4-speed and 3:55s go together like peas and carrots. I'm running 3:55s with a 4-speed in my Demon with a 408 and I think it is a great combo, and I have driven on long road trips with no worries. Of course it's up to you and what you feel is tolerable. Here are some RPM figures based on your tire size (245/60-15) and the three gear sets:

3.91 @ 65 mph = 3215 rpm
3.55 @ 65 mph = 2919 rpm
3.23 @ 65 mph = 2656 rpm