Hi, in regards your problem I have an 85 slabby fitted with an 1100m engine and I run 40mms with an original airbox. I did need to grind down the inside of the frame slight, not a significant amount and certainly not enough to cause problems after that I had to cut two slots in the top of the airbox turned what I cut out upside down and plastic welded them back into place. It's still a very tight fit but is doable. Am going back to the garage later so if I can get a half decent pic will post up for you.