Thanks all - for your great suggestions and advice.
Here is my revised plan of attack.
I am going to cut the miters in my shop - each at 45*.
I am probably going to cut them on the table saw, though I may have to clean up with a router and guided flush cutting bit.
On the larger of the 2 set of shelves, the shelves are each 70" from the corner of the room.
The smaller set are each 38" from the corner of the room.
I will transport them, unassembled, to the job site.
I'll use 1/8" plywood, and a compass to scribe the walls and make a template.
Transfer the template onto the shelves.
I'll be bringing a host of tools for the cuts needed to conform to the scribed lines: belt sander, block plane, jig saw, hand saws, chisels, etc.
I'll use biscuits to keep the miters aligned.
I think that should do it.
I will post pics, and (I am hoping) an account of a successful install after I am done.
Thanks again, all. The check is in the mail....
As @curlyoak suggested, cleaning up the cuts on a large miter with a router & straightedge is a great way to go. I did a couple of wood counter tops with mitered corners. They were about 34" long & with my circ saw & straight edge, I just couldn't get a smooth enough cut. Cleaning up with a router left an extremely tight joint.
A good track saw might be good enough, but the router will make it perfect.