Hmmm.

I'm gonna have to say that a 600 is a bit overkill for your cab dimensions.
They get VERY hot and will likely cause serious heat issues. You've got just about 10.5 cubic feet.

I'm slightly confused...what ballast did you buy? i'm skeptical that a 600w ballast came out cheaper than a 250w.

If you already bought the lamp, i would consider moving up to an actual closet with head room, or building a 5'x5'x8' tent in a basement or something. Even then you'll probably need a cool tube or some sort of air cooled hood. You could then use the cabinet for a veg area.

If you haven't already bought the lamp, i would strongly recommend no higher than a 250w for your cabinet.

Just my thoughts. hopefully someone else will come through and give their 2 cents.

Skeet.