I'd be interested in what boat you're doing this to and what dimensions you are going to vis-a-vis the original rig. This could significantly shift the C of E of the sailplan if the mast isn't relocated at the same time. Details?....
As far as putting the taper in, strength/wisdom issues aside I believe it's usually done by cutting a vertical wedge in the leading edge, squeezing the section together again and welding the resulting seam.
Alden Caravelle. I'm not too worried about negatively effecting the CE. -quite the opposite actually. I'll be adding about 6 feet to the rig and changing to fractional rather than masthead. It has been done before to a sister-ship and I have consulted with a naval architect about the rig change.