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I'm not sure I quite understand this, the slides are always horizontal, but very unlikely to be rated for 500 lbs, can you elaborate?
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I might also not understand this one, but screws are the best bet, even over glue as it appears by "floor" you mean that plywood you've attached to the bed.
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If I am following that you intend for this thing to slide in and out, you'll want to keep the weight down where possible.
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Outside would probably be easier....
Do you have a working plan here with weight distribution, slide specs, designs, drawings, etc?