I'm also at 83.5 / 84 PBH. right on the cusp of 56 & 58 when Toyo was building them in 2003. I went with the 58 & have been happily riding it since then. Started out with a 100 mm stem & Nitto Dream bars. A few years back, I went to swept back bars & a dirt drop stem for a more upright position.
FWIW, the difference in chainstays (44 cm vs 45.5 cm) was the deciding factor. I bought it for touring & wanted max heel clearance. Were I to do it all over again, I would go for 56 for the 26" wheels. I've ridden a 56 Toyo but the difference is & not worth sweating over.
In 2017 (??) I rode a 56 Waterford. It fit well & was very much like my 58 but there was something subtley different that I couldn't figure out. Later I found that the Waterford's had ever so slightly longer chainstays, which probably accounted for improved ride.
A couple of years ago I was at Rivendell & they set up Taiwan Atlantis for me to test. They knew what I rode & given PBH & saddle height, grabbed a demo & set it up. After riding it for a couple of hours, including some off-road trails, I really liked it & the long chainstays did not have effect on handling on tight switchbacks. From the fit, I assumed it was a 56. When we got back to Riv, they told me it was a 53, which surprised me. That was the best riding Atlantis I've been on & what I would buy today if I was in the market.
dougP