I rode yesterday's Ohio Randonneurs 400K in 18:51. That included 2 sit-down meals and dealing with a sheered stem bolt with 16 miles to go (which couldn't be repaired in the field -- could not remove the broken bolt! -- and required finishing the ride with some seriously wobbly handlebars ! How I didn't wreck when the bolt broke I have no idea!). The 400K here also had 14.5K feet of climbing on it, measured by GPS (TOPO USA said something like 18K, but it's consistently off). For refernece, I'd ridden the 300K here (which had a comparable elevation gain per mile) in 13:33.

If you felt comfortable about your 300K experience then I'd get the extra sleep and start later. 23 hours is a long time, and if you did the 300K in 14 hours, you should easily make the cut-off -- that's 9 hours to ride 62 miles if you keep your 300K pace. Some folks like starting rides in the dead of night, but personally I'd pass on the 1am start and get some good snooze.