Hi Bizman, thanks for coming by.
The adjustment element is a 1/4" dia or larger "X" adjusted end - the diameter of the round adjustment is larger than the width of the truss rod slot so, I'd have to remove wood to make it fit anyways. I like your advice of just getting a full length rod, that will take care of worries and ruminations. While measuring the shaped out maple neck back I realized if I wanted 21 frets there would need to be a slight over hang of fingerboard, maybe around .25" or so. I'm fine with that and a full length rod would allow truss adjustment with the neck fastened with tensioned strings - which I would think is an optimum situation for guitar set up.
I found a place that has a good selection of rods, one of those being 19" which would bring the truss rod adjustment right to the very end of the neck heel. I'm hoping that the hex adjustment kind of rod will slip in so I wont need to remove any more neck wood to fit the truss rod. Its time for some shopping now I'd say!
thanks for your comment, its actually pretty nice to be back to this baritone guitar project after having put work into making the neck back form and truss canal. I'm actually pretty excited about this.
Best, - Phil Donovan