Speedy, I sold my 12-speed Univega recently. It had a 42/52 in front, and of course a 6-speed cassette in the rear, with the largest gear being a 28 which was really big obviously. To handle that rear size, I had a Huret Duopar rear derailleur. The shifting worked flawlessly.
I suggest that you stay with a 42/52 in the front, and change your cassette gearing in the rear, with a rear derailleur that can handle that. Perhaps you could do some calculations to estimate the gearing you would like? With the info on the gearing, then you can research the rear derailleurs to see what would work with that size gearing.
After that, if you still need more top end and more low end, then go with a 40/54 in the front, and go from there. Keep on spinnin'.
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