Jim, I've got 6.40s on my '53 J and they do not interfere with the clutch. This is a problem though if the tires are too big. I did have a set of 1970s steel belted tires on my '53 for years. I don't recall what size they were but they did indeed interfere with clutch pedal, especially when backing out of my driveway when I had to cramp the wheels. I installed 6.40s in the early '90s and the first thing I noticed was that did not happen any more. Based on my experience, you should be fine with 6.40s on a J. I can't speak for the Darrin.