Your third option...a vertical with SFB. That would keep the main entire space of the smoke chamber available for food, just as a horizontal/sfb would do, and allow the use of a water pan or drip-catch, if you desired. You could likely fit 3 grates in the space between the bulk-heads, with properly designed hardware for supports. Fully cut the upper bulk-head for the lid, of course. Although, 19" diameter is not really adequate for a very large packer brisket, unless it were a separated flat and point on 2 grates (I've done that a lot over the years)..that does change cooking technique significantly. A horizontal would give you close to 20" or more of grate width, based your 28" tank height (actual bulk-head shoulder has to be accounted for), with a cut-out door opening. 20" grates will fit medium packers to about 15lb. I've fit 18lb packers onto my SV-24 grates (seems I had a monster 22lb in there once, too), being about 22.5" actual width, so I think 15-16lb could be done on a 20" grate.
Overall, with the correct hardware, you could gain more grate space in the vertical design, but suffer a slightly smaller overall grate size. A bit smaller packer brisket would be the main compromise. The flip-side would be to make a full-width door, including the bulk-heads in your cut-out. That would allow for much wider grate(s)...you could add a second grate with hinged supports operated by the lid (similar to a warming rack in a grill), or stationary supports for a fixed grate.
I'd measure the actual distance from the weld-joints on the bulk-heads...then determine what width would be allowed for a cut-out type door (restricts grate width, as well, unless non-removable grate or 2-piece grate is installed), or if I would go for a full width lid. Then, decide whether or not a horizontal would offer enough benefits over the vertical design. A second grate could still be installed, regardless of door opening for the horizontal, and could also be a 2-piece removable grate. A full width lid does not have to be a complete split of the tank into matched size...it could be a 45* cut from plumb (level of grate), and only 1/3 or so of the full diameter as measure on the bulk-heads...or, it could even be a semi-cresent shaped cut-out. There are lots of ways to overcome size restrictions.
Anyway, that's my take, for whatever it's worth...
Don't forget...have fun with the build and the smoking!!!
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Edited by forluvofsmoke - 6/18/15 at 12:35pm