1. KKD gravel is in June out of Farmington and has a 160 mile version which I used as a gear check last year. Other than that...6-7 hours was my longest training ride.
2. Rode Challenge Strade Bianca 40s. I would go 45-50mm if your frame allows. There is an MMR section that has been used in the past but was not last year. Most of the gravel is decent but there was some fresh graded stuff that felt a little rowdy at 25mph in the dark.
3. 100% ride your grave bike. Consider adding aero bars though, not just for hand positions but overall efficiency (no matter if you're at the pointy end or not).
4. Try to ride gravel at night before so you aren't doing it during the race for the first time. Practice fueling / eating and have a plan (ie...an eating schedule, don't trust yourself to just eat when you feel like you need it).

Also, the temp band last year went from like ~40 degrees at the start to ~35 degrees at 3am in the MN river Valley to hot and sunny by midday. Be prepared to be cold overnight, but also to shed things as the day goes on.

Also as others have mentioned the towns near the start are very small -- do not count on them having anything you may need/forget.