Thank you everyone. I'm still following this thread with great interest and I appreciate all your guidance.
I haven't visited the machine shop yet, but only yesterday I came across a possible alternative..
I may have the opportunity to purchase 1 or 2 barrels locally. One is a barrel by itself and in nice condition. It looks like it might be usable or may just require a hone. However, the second barrel and head are in beautiful condition.
The stamping numbers on the barrel are the same as the Atlas project, but it looks like they are for a Commando, not Atlas?
Neither barrel seems to be "spigoted", which I understand is where there is a lip on the head of the barrel bores that fit into the grooves of the Atlas cylinder valve cavities in the cylinder head. The second barrel/head looks like it has been newly re-bored or honed and is in very nice looking condition. I'm not sure if those valves look right, but I will get the chance to see it again soon. 2 of the valves look to have seats and the other 2 don't.
My question is, would these barrels and the head fit the Atlas crankcases or would there be a lot of other things I would need to consider?
Thanks again for all your help. I'm very grateful.
Here's a video containing a few clips and photos of the two barrels & head.
1974 Norton 850 Commando MK2A
1964 Norton Atlas 750 ("Shep" project bike)