Charts are showing an 18" seat on Western Saddles for the specs you posted.
One reason 17" Seat are the maximum size as standard is because of the length of the bar in the Saddle Tree, if the bar is long enough to accommodate an 18" seat, care must be used in selecting a Horse with a long enough back, too long a bar on a Horse and too much pressure is put on ribs that are not designed for such weights/pressures.
Going custom means someone will be talking to a semi or custom saddle maker, this insure both the rider and the horse will be happy.
I would suggest a Saddle with an A-fork or Wade type Swell as the narrow fronts offer a touch more thigh room also.
I ride in a 19" 36cm Duett saddle - my mare has a short back (too short to accomodate a traditional western saddle, much to my dismay as I would love to try cattle penning with her and don't trust myself to stay in the Duett!) and it is not too long for her.
OK - I am 300lbs 5'7", and my 17" Circle Y Park and Trail is actually a bit big.
For several years we had an 18" custom saddle for my husband, but it took a really really big horse to use it on - actually the only horse I ever owned who fit it was a full draft. She was an 18.3 hand Shire and it fit her perfectly. I bought it used and it was made for a QH??? Ok, but like I said, it was just huge.
Look at the 17" saddles and give them a try, you might be surprised.
(This one is for sale as it no longer fits my horse...)