I have a peice of green Mahogany 1' x 2' with the bark still on it, how do I dry it so that it won't crack across the grain?

Nice score!
1. Get the bark off it. Is it a nice straight-grained piece?
2. Paint the cross-cut ends with wax, use a torch to gently melt it in. This will help to even out the rates of water loss from the various surfaces.
3. Outdoors, under cover (not tight-wrapped) where you are, you can expect the wood to dry down to a stable moisture content of 12 - 14%.
4. Estimate 1" per year. eg = a 4" dia log will stabilize in 2 years.
5. There will be some checking, nearly impossible to avoid. However, trim that off, say nothing, and you'll look like a wizard. In doors, you can probably wreck the piece in 6 months.
6. Weigh the block and keep track of the weights (every 3 months). It's done when that levels off.