I would space the wood on top so water will shed after a rain or you spill your beer.
You could space the boards about the thickness of a pencil. This would allow the wood to expand and contract.
I would pre finish the boards before i attach them. It will be easier to finish them on all sides. Time well spent to extend the life of them.
Screw them down also and if you need to refinish in a few years it will be easy to remove and replace.
The overhang...you could let it extend beyond the table to create more work area.
Just round off the corners, put a semi circle on the end. I think there are a few pics on naked whiz site that are done like that.