As a datapoint, I am 5'11" ish, purchase pants that are stated to have a 34" inseam and have arms that are somewhat longer than normal. My ride is a BMC SLR-02 model year 2015 that is 58". The 58" fits me perfect and after transitioning from a bike that was too big would really be concerned on riding too large a bike.
I think the geometry of the SLR-02 and SLR-01 vary slightly and don't know if designs have changed meaningfully as far as fit in with later models.
At 6'-3/4" you likely need a large bike than me, it may be that your in between sizes though, but my experience would tend to suggest too small is better than too large.