Personally, when I kept my adult bp in a large cage he stopped eating. When I put him in a cage that was 3x18x8 (a rubbermaid) he started eating like a champ again, same thing with my other bp who was much smaller.
I wouldn't keep a bp in a cage that big, they like small, tight, areas. Maybe something about 3'x1.5x1.5 would be better.
As for the varnish and what not, it is best to do it in warmer weather or it won't cure. plus you should use aquarium sealent on the joints or it will seep into the joints and warp the wood. You could always use melamine (heavy, but works and is easier than having to put several layers of some kind of varnish) usually the kind that is waterproof and for kids, as it won't be just water going onto the wood, it will also be urine and fecal matter from the snake.
Whatever you do don't forget to put venilation on the cage, but not to much as it won't hold humidity. I also wouldn't recomend using glass, try to use a nice strong plexi glass, it is easier to drill through.