The 6x6 anchor has a 7.5x7.5 base by: Editor-Rich Bergman
If you are considering using the post anchor as a support post base for a compression load as some people seem to be doing, then the 6x6 post anchor may work.
It depends on the actual size of your 8x8 because the 6x6 anchor has a base size of 7.5"x7.5". Sometimes 8x8's are milled a lot smaller than a true 8x8 and if so it may fit within the 6x6 base size still leaving the corner fastener holes exposed.
In that case it would work fine as a support post base for and 8x8. But you will have to check the actual size of your 8x8s.
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