Sorry for this late response -- I just noticed this today. If you plan to make a "drop in kit" for this modification, that must mean you're keeping the original axle size, which is a bad idea.
The stock bearings in the Norton rear axle are metric -- 17mm, which is about 0.67 inches. Norton sleeved these down internally to fit the 5/8" rear axle which is 0.625 inches, probably because they had a lot of rear axles and wanted to use them up (Norton didn't have much money during this period and never spent it unless they had to). So if you're offering a "drop in kit" for a one-piece axle, that means you're going from 2-piece to 1-piece, but keeping the puny 5/8" axle.
When I converted to a one-piece axle on my bike, I got rid of the sleeves in the bearings so they were 17mm again, then I fitted a 17mm rear axle from a Honda CB 350. (Many other Japanese bikes of the period used 17mm axles, so they're not hard to find). Then I enlarged the slots in the swingarm to fit, which can be done precisely without removing the swingarm by using a big file and a stepped bar with diameters that slowly approach 17mm so you can keep trying it in the slot and know how much you're taking off.
The fact that Honda would choose a 17mm axle for the 350, which is much lighter and less powerful than a Commando, should be enough proof that when you convert to a one-piece axle you should also convert to the bigger axle that the Commando's wheel bearings were designed for.
And the way I did the modification, I didn't have to make many new pieces but only had to drill out the dummy axle and the various spacers to 17mm on my lathe. The only tricky part is the speedo drive, because the "top hat" sleeve that goes in the drive is too thin to drill out. So you make a new one that's 17mm on the inside, then bore or file the speedo drive to fit it.
