Anyone ever add one to a fly rod?

I've never built rods, thus my question.
My concern is that once I remove the butt cap from the reel seat, the blank will be to close to the end of the seat. That won't give me enough room to install the threaded butt cap that the fighting butt will screw into.

For those that build their own fly rods, generally how close is the blank end to the end of the reel seat?

I'm just trying to figure out if I can do this before tearing into a perfectly good rod.

Thanks for your input.
SF
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