George,
You're 100% correct. 6.5" backspacing (ET 13) is the optimum offset for a 17"x11" with a 335 section width tire. It just isn't safe to go further inboard and it will stick out a very small amount. If you go up to an 18"x11" you can go further inboard because an 18" rim clears the obstruction that interferes with a 17".
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