I have a similarly built horse (Lusitano x Trak). The full front gussets help her. I ride in a 17.5 inch Borne Sankey.
The Fidelio is a hoop tree and has a little bit more longitudinal shape than the Largo which is a more open seat but also flatter longitudinally. If you horse is truly flat front to back, The Largo could be a better choice but does have a wider twist feel to it.
A big thanks to everyone for their input. Really appreciate it and I’ll let you know what I decide upon. 
We have used a Fidelio on our mare. Trainer bought it for 2 of her horses, then of course saddle fitter 3 comes in and says NO it doesn’t fit your horse. So she loaned it to us for our horse who had started objecting to her saddle. Our mare def liked it better (arab, short back, round, forward girth spot, big shoulders). My DD said its not a bad saddle, not as cushy as the Customs, leather quality is ok, nothing special, def not same quality as Customs. But it doesn’t cost as much and they do fit the 55 gal drums on 4 legs well. We may end up buying it from her if mare still likes it (she was on layup for 2 months so not using it).
I found an older Rembrandt which fits him well. The seat is a bit small for me but I’m losing weight so fingers crossed! It doesn’t put me in a bad position and he seems happy and his comfort is more important than mine.
I tried a Custom Coronado, a Schleese HK, he’s okay with an old Wintec but everything else didn’t work. Finally just bought a Moffett Vogue…treeless. 3 rides in its amazing! Very secure feeling and made my posture/seat almost automatic…something I was struggling with in the Custom and Schleese. Someone mentioned the best saddle is the one you don’t notice…I think I found it in a Vogue! My horse is almost 16’ 2" and wears a 72 blanket…to give an idea how short bodied he is.
(I am stuck in posting limbo, but in case the mods can send this through…)
I have a Black Country Kur model with upswept panels on a XW hoop tree for my super wide, flat backed and incredibly short backed little mare.
