Kev, go ahead and drill holes if you want authenticity, but I can tell you from experience that it's a buggar of a job to install the Nyloc nuts inside the air box, and they really don't serve any purpose. As Grant says, the carburetor concertina rubbers push the airbox back against the frame tubes as much as is needed, the bottom brackets can also be eliminated but it's kind of nice to have them to feel safe. I can tell you that if the airbox cracks it's very hard to find an adhesive that will glue it back together. Mine cracked from the holes at the bottom and I really couldn't glue them back together properly, had to bond a large patch to the inside with silicon adhesive, really couldn't solvent weld the plastic at all.
Ken