I've got a set. I'm running the JJ at the frame, and yeah it is a little wider than the mounts, maybe 1/8". I had to beat the mounts open a little with a 3lb sledge. That was a little frustrating.
They're really well made and I think they're the best option out there for a daily driver, especially one in the salt belt (no heims). Having adjusted mine three or four times to get the pinion angle exactly where I want it, I do wish they could be adjusted without having to take one end loose. Adjusting them isn't that bad, only takes like 45 minutes total, but it would be a lot easier to just loosen a couple of jam nuts and spin the tube.
That said, I'd still buy them for my application over any other brand, and I say that having waited like 3 months for them to do a run of them.