This seems like it oughtta be easy to figure out, but Imma defer to more experienced mechanics:

I have an old Bridgestone 600 road bike circa 1985. 126mm rear hub, originally a 6-speed freewheel, but it got chainged to a 7-speed freewheel a few years ago. Not sure what make/model chain is on it at this point, but the SRAM quick-disconnect link says "8-Speed"

Recently the mechanic at my LBS attributed trouble I was having shifting in front to worn teeth on the chainrings (which makes sense, since they were the original parts with 25 years wear&tear on 'em). He took the liberty of replacing the entire crankset with a brand new (2010 model year) FSA Gossamer MegaExo crankset

...and ever since, the drivetrain has been extremely sensetive to cross-chaining. I can only use the three smallest cogs with the big ring, or the three largest cogs with the small ring; otherwise I get major loud grinding/scraping noise emanating from the crank area.

It seems to me the problem is that an 7- (or 8-) speed chain is wider than the 10-speed chain that FSA anticipated would be used with their modern crankset, and so the seemingly modest chain angle causes the side of the chain to rub against the teeth of whatever chainring is not being used.

Does that sentence make sense, and does my analysis seem likely? (If so, it seems like the solution is to simply put a modern 10-speed chain on this bike.)

Thanks.