Yeah, Elmer, that is a good bit of spine to remove. I don't normally try to remove more than 5 lbs spine, but if you get into the middle of the shaft, it can come down pretty quick. I'm looking to make a rig to do a much longer taper, like 24", and I think a ten pound drop would be realistic. I have extended my regular 10" taper by hand and can drop the spine pretty good and the long taper makes a really nice shooting arrow.
I used to make a 10" taper with a small block plane and it worked very well. It should work at 24", too and would definitely bring the spine down.
Have you confirmed the accuracy of you spine meter? An XX75 2016 will spine at 59 lb.
You could also leave the shafts a little long or go up in point weight to make use of the stiffer shafts.
Rick