Some good advice here as usual.
378k in 16.5 hours is pretty fast. I'm normally just under 6 hours for 100 mile training rides, and 20-21 hours for a 400k. You just get way slower as the miles add up. I don't think I could do 378k in 16.5h solo. My only chance would be to find a fast/but-not-too-fast group and do as much wheel-sucking as possible. It's tricky because you don't want to run out of gas either. I took a 400k out too fast last year and had to stop for a nap at 300k. The guys I was riding with finished in 18h, which would still be just off the pace for your ride. I ended up finishing in 21h after a 2 hour nap on a picnic table. Good luck!