3/8" is huge for any shaft. 60-65# fir shafts are typically found in 11/32" size up, and perhaps up to 23/64". Have you actually checked the spine? I'd bet it's 90 or pounds.

Some pencil sharpener type taper tools might accomodate a 3/8" shaft, but the length of the taper will probably be pretty short. I prefer to use a pine jig that I made for my benchtop disc sander.