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IMHO anything less than 5" looks weird, unproportional look. but its what you like is what counts, just put the stack in the bed first and get a good look at it and make sure you like it before you cut that hole.
Not what your looking for on a pick up but my single, 4", non chrome (triple uncool eh?) on my F-Superduty. Sounds awsome.
If I was going to do a stack that is what I would try. I don't want to waste bed space and I have a cap that I put on mine. I would have to have it stick out a little more to clear the bed side though.