The back pressure thing I don't understand, IMO it's a bunch of nonsense, like how 15-20 years ago the common knowledge was that a turbo NEEDED back pressure to work properly which is BS. Back pressure makes an engine work harder, costing efficiency. And really, an engine turning ~2000 rpm at 20% throttle opening doesn't care if you have a single 2.5" or 3" or dual 2.5", you wouldn't have back pressure with any of them since you aren't flowing enough air/exhaust to max any of them out and it would be a wash from a mileage stand point IMO. Where you will notice it is when you do want to play (which you will) and get on it.

BTW, a single 3" pipe has less surface (7.065 sq in) than a dual 2 1/4" (7.95"), but the 3" has less boundary area than a dual 2 1/4" so it's probably a wash and they flow a similar amount of air. Remember, 250 hp Dakota RT's came with a single 3" system from the factory (along with the 98 and later rams, they may have 2 3/4" systems though)

Your exhaust guy may be feeding you a bit of a line about "back pressure" cause a lot of places don't like bending anything bigger than 2.5", go with the 3" single