Dear Brian,
Thinking more on your tapering problem, I seem to remember that the extrusion needs pressing athwartships before cutting the taper.
We used to use a set of padded steel "crocodile jaws" that were set in the press. These took the shape of a pair of adjustable, pivoted arms jointed at one end and set the correct distance apart to take the section arranged athwartships with the hinge pin just below the start of the taper and the other ends free to close-up under the influence of the press.
After gentle pressing, with lots of checks to see that you are not distorting the luff groove too much, you end up with a section that is tapered athwartships and flared out fore and aft.
With care, and if the section is stable, it is possible to reduce the athwartships dimension at the tip to very close to that of the width of the luff groove reinforcing web. This is how we configured the Flying Dutchman and Flying Fifteen masts at Alspar way back in the mid seventies! We had a real job finding room for the halyard sheave and in the end just formed a horn on the top not unlike a longbow's tip that a loop in the head of the sail just hooked over.
It is easier to judge the size and form of the wedge to be cut out of the front of the section, and a LOT easier to close the two sides in, maybe with the press again.
After cutting, but before welding, is the time to straighten out any curve or fore and aft bend in the back of the section.
Good luck again,
Regards,
Joe Henderson.
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