Smoke stack placement help

[fdtruckie]

I'm currently working on a 275 gal. offset, reverse flow oil drum smoker.  I have one dellema so far having to do with my smoke stack placement.  My stack is 6" pipe and will protrude into the top cooking grate if I place the bottom of the stack near the bottom cooking grate.  Which is no big deal, I'll just notch out the top grate to fit.  Or do I place the bottom of the stack at the top grate?  There will be about a 10" difference.  

Thanks