I would weld it up and try it. I personally would't make the CC hole smaller. If you need too you can close the damper on the SS to reduce the draw. If worse cones to worse you can always cut the stack 6" after it is welded on. jm2cs worth.
I know what you mean wanting the stack to be longer, I like the smoke above my head too.
I personally would get some 4" or something that I could match the pit calc and my desires. But if I couldn't do that then go with BC.
Making memories.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
If you're hellbent on using the 6",then go for it.
Like bluecatfish said,you can always close the damper some.
I used a 5" round on my 120 and calc. calls for 25" stack.Perfect,in my application.
Tall enough to keep the smoke out of my face,but short enough to fit in the garage.
And at that angle,it holds the door up.
Now,to figure out some kinda spring hinges for my heavy old door.....
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Problem with to talk of a stack is potential cooling of smoke that can cause condensation dripping on food...may not be an issue but 14" sounds pretty short to me...jm2cw