I've run Sand Mountain here w/ just my pipe, stock bore & 13:1 compression in high 3rd. Someone with the same skill but 40lbs lighter could have done it in 4th. You need to get out there and figure it out. Folks can give you a general idea, but if you're say 40lbs lighter than me, you may do better geared differently.
Screw gearing charts. They don't/won't apply to all cases i.e. wet sand, etc. Countershaft sprockets are cheap. Start w/ a 13. The rears are more expensive, but older banshee sprockets fit and are much less expensive BUT you need to entirely remove the axle to install them. I've found that I run fastest here w/ 14/41 in dry sand.
I ran all sorts of configurations thru a gear chart one day and what it said was the fastest wasn't.
I'm ording some purpose-built sprockets this week - one's that go on w/o taking the axle off.