I think I've gotten it all sorted and managed a test ride with only a single, self-inflicted issue.
Here it is on a 5-ish miler this morning:
Getting the rear shifting to behave came down to inserting a small piece of shift cable liner in the cable guide at the bottom bracket. This significantly reduced the friction of the whole system.
Gearing is still taking getting used to, as its a bit steeper than I typically ride and with massive jumps. I'm both getting older and spoiled by my more modern drivetrains, I think.
There's plenty of clearance with these tires (27 x 1-1/4" Paselas), and I suspect I could go up to 38mm if I switched the rims to 700c. The main issue is getting the rear wheel into the dropouts - the conflict is with the chainstay bridge. You'd need to have the tire under-inflated.
Bike handled fine on the groomed gravel trail I was on this morning:
which leads to a nice overlook of the Colorado Front Range:
I took a paved descent back home and found the bike to handle much more comfortably in the drops. I almost always ride on the flats or hoods, so that will take some getting used to as well.
One more pic of the bike itself:
And the issue? I forgot to correctly route the drive-side toe strap through the top loop on clip, and so it got loose on the descent.
Also, I must confess to one sin - I bought some brown Cane Creek hoods to replace the black Dia-Compe hoods I had on hand, as I thought the black levers, hoods and tape was too much.