from what i have read, you will lose low-end torque using that setup, not enough back pressure...but i'm not an expert in this subject, so what do i know for sure that's if you run two pipes, if you run the y out of the single, you might not.
I would keep it simple single 2.5 in dual 2.25 out muffler. Thats what I have dont know if I gained or loss power feels the same just louder. Keep in mine that if you are goning straight out the back by the time your pipe bender gets done you will gain back the back pressure. I also got a smaller spare 215/15/65 so I have a spare and duals and you can always add mods. Throw a S/C on with some fuel mods and then a smaller pulley then WI and so on and so on. What year truck what model? If you want realy simple keep the factory muffler and y it. The only thing you will need to watch for is the weight valve setup for the rear brakes. On the lift it looked ok but when we put it back on the ground it was rubbing. My fix was easy just gave me an ecuse to raise the truck 2"s