Try an NGK B5HS or B6HS and gap it properly to the .016..... They are super hot plugs...I have had some luck with them with a similar problem on my 78 Maxi.
I would like to see a picture of this bike if ya don't mind. Sounds cool. I've always like Franken-peds.... its awesome that we can toss stuff together and build something that almost or does run.
I've had plugs last me 2000 miles or more with proper mix.