bbq conversion to a reverse flow

I think you'll be alright with a larger firebox (I did the same), as long as the air intakes can always be closed off, not a problem there either.  Biggest item is to make sure the firebox to chamber opening is correct, if not a bit over.  Then secondly, make sure that at the other end of the RF plate, that opening is at least as big as the firebox to chamber opening, preferably a bit bigger.  This will insure that you'll have proper air flow and draft through the cooker.  Yes, the stack must be moved.  How far is your RF plate from where your bottom grate will be?  Go at least 4".  Breaux Bridge huh? Is Gators Cove still open?  My son played a number of select baseball tourneys a few years ago over in Breaux Bridge and we always went to eat there on Saturday night.  Good crawfish, matter of fact, everything was good.   Good luck with your conversion.

Total length, but for a RF, you don't need the stack to drop down to grate level inside the chamber, just be flush with the outer surface of the chamber.

I did the same as Bruno and built my firebox bigger after I finished converting my little brinkman pitmaster to a reverse flow. I got carried away doubled the size of the cook chamber. Here is a link to my build http://www.smokingmeatforums.com/t/98141/double-deluxe-pitmaster-mod  . This link doesn't show my new fire box or the mod on the inside to move the original chimney over next to other one. I used flexible aluminum dryer duct from chimney across the back on the inside so it would pickup next to new chimney for a true reverse flow.

You like like you are on the right track so far. Keep the progress and the pics coming.