I'm planning on running 5 inch stacks, is it better run y pipe or 4 inch square tubing to dual stacks, for best sound and even smoke distribution?

 

If u do a tube, make sure u put in a divider to push the smoke even. So one stack dose not out smoke the other.

 

I never had a smoke distributing problem with a 6" box. I even came up in the box all the way over to the right - straight up where the exhaust runs from underneath. The smoke will equalize in the box then come out even.

 

My stack setup is like this and I get a very even smoke distribution out of both.
On My flatbed, I welded up a nice 3 inch tall, by 8 inches wide and 8 feet long.
I made a 3 inch hole in the dead center of the box, for the factory exhaust. I ran a 90* elbow with some bent 3 inch pipe up into the box.
Then I set the first stack where I wanted it the made the other the same distance from the center as the other. Then I scrounged up a 5 inch peice of pipe and welded it onto the top of the box and put a 3 inch hole in the center of the 5 inch pipe. Then I slid the stack down onto the pipe and bought a chrome clamp and clamped it on.
The reason for the 3 inch hole coming out of the box and into the stack instead of 5 inches. Was in case one stack smoked more than the other, then I would simply open the stack that smoked the least up some more to increase flow in that stack.
Like I said, it worked for me and I never had to open any of the holes up more!
I love it, and it was very simple to do.
Hope it helped ya, and did not confuse you!!!
Wheelo