Keep belly flat for this, but better to trap the back.
If it was me, I would crown the back, and just give the belly round on corner.
If you do again, for same weight/length, make it longer. Start with 80", you can make shorter later....
I would back with maple wood, this is much more better combination. I think good birch works well too.
Ash wood may be too strong, but its an option. If using maple/cherry, I would make oval cross section and fully round towards tips.