I'm tackling this issue as well on 68mm BB shell. I have an older Exage M500 Shimano crankset , it's way too far in-boasrd,, and the spindle I have for it is 2mm longer than the one it's supposed to use. I have decided to go Sram , Their crank spindle boss is practically flush with the small cog, where the Shimano is recessed by quite a lot. I know this isn't a scientific approach, but I can always get a shorter BB, which is easier than starting with your small ring slammed against the frame and messing with spacers, which i don't like the idea of using in an area that is prone to creaking anyway.

Shimano

SRAM