Ok so stacks are a must, that has been well established. I would like to keep $$ down as much as possible (as most everyone else) so I am looking at the "smoke box" option. Question is if I put my 4" pipe into the bottom of a 6"x4" tube: will the smoke roll even out both sides? I have looked at splitters in t pipe and wye pipe but those dang things are spendy. Also what are you guys using for a 90*. Flex pipe with clamps or solid pipe?

 

Check mine out here. https://www.dieseltruckresource.com/...d.php?t=114391

I'm with you on expensive exhaust parts. I have no splitter, the pipe feeds near the center of the smoke tube, and seems to roll smoky evenly.
My system is this, BD Diesel 4" downpipe, shortened about 2 feet for the Dynomax Bullet race muffler, and 90* elbow, which is turned towards the center of the truck. It feeds a 45* elbow which meets the short flex pipe on the bottom of the smoke tube. I guess I need to get a pic from a little further away underneath to show the way I did it.
On top is a pair of short 5" pipes, flattened slightly to connect to the tube and the stacks on top of that.