Since you are going to change out the stack anyway, just for an experiment start reducing the length of the stack, tack some readings and post for everyone would be good information to have. Just a thought.
Another thing is and you probably already know this, but a lot of first time pit builders may not. When sizing your stack there is a difference in ID and OD, I think some people find a piece of pipe run their tape across the end to get the size, If you are measuring lets say 4" pipe and you measure across the top, if your pipe is 1/4 " then all you have is 3 1/2 " ID pipe. The only reason I would bring this up is I read a couple post on new builds where that was used. So be sure and check the Inside dia.
Gary