I went the helicoil route many years ago. When I owned a bike shop in the early 80's I converted one triple and two double Avocet cranksets into one tandem set (I got them at a great price, so the cost was not an issue). I drilled out the threads of the two double spider arms and one "left" arm, and then used helicoils and taps to rethread each arm. We had the helicoils and taps in the shop for the all too common stripped crank arm repair. After retaping, I epoxied the helicoils into the arms, waited for the epoxy to dry and then filed off the excess helicoil. Worked well and looked good. Lots of work, tho'.
Kevin