Sorry that I didn't get back sooner.
To answer some of your questions in no particular order:
The crankset that I put in is a Mundial 52/42 double with the midpoint-point being the 42mm front chainline that I've already stated.
I took a string, as advised here, ran it along the inside of the outermost (right-side) chainring and found that it pretty much lined up (+/-1mm) with the top of the 4th cog which is the middle one of this 13-15-17-19-21-23-25 7-speed Shimano Hyperglide cassette.
Getting my hands on a copy of "Zinn & the Art Of Road Bike Maintenance" and a caliper, I made some more measurements. With a gap between the cassette and the right dropout (6.5mm) and, using the specs for the cassette and the 135mm OLD, I calculate that the rear chainline is 45mm.
BUT...
Taking a second look at the rear triangle alignment, I estimate that these are off-center by 4mm towards the left (non-drive side). This current situation would be even WORSE if the triangles were properly alligned.
The width of the chain is 7.8mm.
There must be something else out of whack or my measurements and estimates are off. The triangle misallignment should actually COMPENSATE for the difference between the front and rear chainlines. Everything, except for the triangles, should line up.
I took the bike out for my first ride this year and am finding that it's handling quite fine although, with the stop-and-go cycling of my normal commute, I only had the bike in top gear (that is: large chainring to smallest cassette cog) for literally only a few minutes.
The only reason that I'm believing that something's awry is the distinctive sound that I recognize whenever I BACKPEDAL in this top gear. It's a sound that I'm familiar with from a lifetime of using friction shifting: one of an adjacent cog starting to catch the chain as you slowly shift gears but not yet grab it enough to actually shift.
The current triangle allignment has been the best that I could do because when I tried to fine tune this I would always overshoot the amount of bending that I wanted to accomplish. I'm now thinking that the way to do this is to get a bare rear-wheel axle, put on some nuts set to the proper O.L.D., and anchor the triangles together for greater resistance when I try bending these with a wood 2x4 as per Sheldon Brown's instructions on his website.
My estimate for the proper bottom bracket spindle length will be just over 120mm. Where can I get one that is ISO-square-tapered that is under $40? The Campagnolo 70-SSx3 (124mm) should work but I don't know where to find that. What else is there that is comparable? What I'm finding online is either J.I.S. or unspecified.
A complete cartridge bottom bracket may have to be for a 73mm housing. My bike has an english-threaded housing despite being 70mm wide making it incompatible with the Italian-threaded ones.
Many thanks.
(I'll probably end up leaving the bike alone for this year and do all of this when I once again overhaul my bike at the end of next winter. Swapping the cranksets was something I first contemplated over a year ago. I waited until I found a good deal for a crankset on Craig's List.)