I have an 83 Super Mirage, which is nearly identical - just one notch up the ladder. Your crank is probably a three-piece, so just take it apart & use smaller single-stack bolts for the inner 42t crank.

You might get a close chainline by flipping the spindle so the "short" side is on the "drive" side - this will bring the crank closer to the BB. I just did this successfully on a Schwinn Le Tour and got the chainline within 1-2mm.

This bike is full of odd threading - mine has a French-sized stem, outer seat tube diameter, and a Swiss or French BB. Unfortunately I love the paint & style so much that I keep messing with it, despite the higher weight of 2040 hi-ten. Post pics of yours when complete.