No beer ??
Jim, looks like you and Mr. Spokeshave are going to be new best friends. Check your references, I do not believe the mast began tapering until above the boom jaws.
Find and mark centers on both ends of the dowel, then mark the required radius at the truck end. Then mark circles on the dowel where the taper will start and at each point where the radius reduces by 1/64".
Then fasten the dowel somehow so it won't get away from you. Take the spokeshave and set the blade for 1/64" or even less. Run from where the taper starts to the mast truck, then again from your first 1/64" mark, then the next mark, etc. until you have a tapered flat on the dowel.
Turn the dowel over and repeat. Then rotate 90 degrees and repeat again, last turn the dowel over once more and repeat. You should end up with four flat tapers on your dowel.
Now take the spokeshave and plane down the remaining round parts (between the flats) until you have an 8 sided taper. At his point you will have planed off your depth mark references but that is OK.
Now plane down the 8 points where the flats come together to make 16 sides. Now you're ready to sand the dowel round.
Sounds like a lot of work but this method will ensure your taper remains centered on the dowel as you work.
Enjoy!
Bruce