The 64 is a good enough starting point for a cruiser with it's heavy flywheel. My 68 is a decent enough highway bike and it still has it's light flywheel, but I am considering changing to the heavy one for the upcoming rebuild.
My 68 has the 78 gearset fitted and I heartily endorse the swap, it is the smoothest shifting, shortest throw, most positively engaged gearbox in any British classic. I use the 20T sprocket.
On the rear wheel, fit a 43T sprocket/drum combo. These are about $120 or so, but you get a new drum surface and new sprocket teeth, so fit both new brake shoes and a new chain at the same time. These two changes will dramatically alter the road manners of your bike. Adding a 750 kit is a great idea, as is fitting hardened camshafts, in the case of the 750 kit, I would consider either the Megacycle 51060 or 51015 ( replica of Jomo #15 cams....these are in my bike, with 500 R tappets and special pushrods. A light valve train is always a good idea for longevity and for security.... a floated valve can ruin a ride.)
The DLS Triumph/BSA wheel can be fitted to your forks, but I would use the later, wider forks from a 68-70 650 or Trident, or from a 68-74 500, although most of these had the rigid mount top yoke, which I would change for the rubber mount bar type of the 650s and Tridents. These forks are the "shuttle valve" damped type, with damping on both the compression and rebound strokes. New hard bronze bushes and new springs and seals will make for a nice set of forks and these DO bolt straight on to your frame. New hard chromed stanchions are available as a nice upgrade.
The disc forks do not bolt straight on (unless they came from a Trident), as they use different bearings...you need the adaptors that the Trident's used.
If you are into the engine, consider changing the con rods to the later, thicker ones found in the 68 and later 650s and then balancing the engine.
For security, safety wire is a little cumbersome and a bit of a hassle, I use loctite...there is even a type which will wick into already assembled nut/bolts. If you have a nut and bolt situation, you can use NF bolts and locknuts...I favor the Jet or Kay nuts, as found on aircraft, as they are quite strong, self-locking, and have a reduced hex for easier access.
My bike put on 85K miles.....and the engine is still running in another bike...a friend is using it while I rebuild his TR7 engine. The chassis is being powdercoated. I will be pulling the engine down and doing a report on it's condition early next year. I'll just say, I suspect it is in pretty good shape, from what I have seen of it while doing the engine swap. I have used synthetic oil, mostly Mobil One, for most of it's life.
I was able to run at 75 to 80 mph all day........rather the BIKE could, but I am in the stop and walk around a bit every so often group. Old guys aren't quite so durable, unlike well set up old bikes.