My FG conversion is an old MTB, too, so I ran into similar constraints. It was originally designed to have a 48-38-28 crankset, and 45T is about the biggest chainring I can use without scraping. If you have to use a 42T to make things work out, it won't be the end of the world.
BTW, here's the link to the Sheldon Brown blog post about UN54 bottom brackets:
Shimano UN54 Cartridge Bottom Brackets Relative Chainline ? Harris Cyclery Blog
I was bored one afternoon and did the rest of the math to figure out the NDS lengths and by how far some of them were asymmetrical (I figure a difference of 1mm or less is close enough to call symmetrical
):
Code:
Bottom Bracket/Spec Actual NDS DS Symmetrical?
Sugino/Tange 68-103 103mm 17mm 18mm Y (+1mm on DS)
Shimano UN54 68-107 108 20 20 Y
Shimano UN54 68-110 111.5 23.5 20 N (+3.5mm on NDS)
Shimano UN54 68-113 114 23.5 22.5 Y (+1mm on NDS)
Shimano UN54 68-115 115.5 24 23.5 Y (+0.5mm on NDS)
Shimano UN54 68-118 118.5 25.5 25 Y (+0.5mm on NDS)
Shimano UN54 68-122 123 27 28 Y (+1mm on DS)
Shimano UN54 68-127 128 30 30 Y