i can't decide if i want 6' or 6'6". can i trim 3" off the butt of a 6'6" spinning rod to split the difference? how will it affect the action of the rod, if any?

3" inches off the butt will change the action slightly, but only slightly. Probably not enough to notice overall. Keep in mind that you are likely to void the manufacturer's warranty, however, if you alter the blank in any way, other than just building a rod on it.

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which way will it alter the action? stiffer or lighter?

Action is not "stiff" or "light". Those terms refer to power. Action is flex profile. Shortening the blank from either the tip or butt will make it less "fast". Meaning, that a greater percentage of the rod will bend under load. Cut enough and a fast action rod will become moderate, and a moderate action rod will become slow.

Cutting from the butt also reduces the available power of the blank. If you cut a couple feet off that blank you will have a less powerful blank (remember thant most of blank's power is in the mid and butt sections). But 3" is not likely to alter either by very much. I would say that in your case it is almost a non-issue. 3" off the tip, however, is another matter and will greatly affect the overal feel and purpose of the blank.

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thank you very much!