What exact model is the GT crankset, there is a double and a triple. It may be marked on the inside of the arms, like "TGT" .

I know with a '83 Stumjumper and the Sugino AT triple(straight arms) , a Shimano UN72 in a 127mm was perfect. The original spindle was a 124.5mm.

The 3rrb is supposed to be 136mm, which would be really wide ! The AT arms are straight , as are the GT, so it should work with a 127 too(assuming it's a triple). I've read of others using the 127 with 1-2mm spacers on the driveside. Cranks do not need to be perfectly symmetrical either. The only longer BB's than that are Phil Woods, is it worth it for these cranks ?

Don't buy a Shimano cartridge UN54/55 either, they are not spacer friendly . A Tange LN-3922 would be best.