Not to disagree with any of the other list members, but I'd be nervous about 42 mm valves without re-angling the intake. That's pretty much the same size (or slightly larager) that the factory used in the big valve heads, both the AMA homologated head for the standard 750 and the nead for the short stroke 750. In both cases, they changed the intake valve angle from 28 degrees to 26.5 degrees to get more clearance between the valves. All the other Norton head builders I know of, including Dunstall, Mick Hemmings, and Steve Maney, re-angle the intake to use that large a valve. C.R. Axtell used to fit 1/16" larger intakes with stock size exhausts without changing the valve angles. He said that anything larger ran the risk of valve interference with his cams (which were pretty mild, pretty much the same as the PW3), especially when he ground them with close lobe separation (102 degree). He liked the closer lobe separation for the race engines he built, but sold most of his customer cams with 105 degrees lobe separation, mostly because he was worried that the buyers wouldn't be careful enough to check the valve-to-valve clearance, particularly with larger valves.
If you do choose to go with the larger valves, I believe you'll need to replace the valve seats with larger diameter ones. As I recall, the stock seats won't accomodate the 42 mm valves.
If you choose to go ahead with the 42 mm valves without re-angling, be sure and check that you're not close to valve hook-up during the overlap. I like to keep .060" safety margin between the valves at closest approach. If you're really close, you can pick up a little more clearance by sinking the valve seats, but I dislike doing that.
Hope this is info is of some help.
Ken