I have just restored a '68 D-18. The guitar was "upgraded" a long time ago; scalloped braces, enlarged soundhole, replacement ebony fingerboard. When strung up with medium strings, the neck has a pretty severe upbow. I did a refret with wide-tang fretwire, which did not help. There is not enough fingerboard left to do any tapering of the thickness. The fingerboard is glued with white glue. I think the thin fingerboard compensates well for the chunky neck, so I would like to keep it if possible. I am thinking that I have two options; either reinforcing the square tube truss rod with carbon fiber and epoxy, or installing an adjustable truss rod. What would you have done? Obvously, the carbon fiber soulution is the easiest, but will it provide sufficient stiffness to straighten the neck? Can I install a Martin style adj rod without having to remove the neck? The neck angle is pretty good as it is, and I suspect the dovetail joint might have been epoxied at some point... If I go for the adj rod, will it affect the value of the instrument? Thanks for your time!