I'm running XX1 BB30 crank in my old-school steel frame.

XX1 BB30 Crank in 68mm BSA Bottom Bracket-photo-2.jpg

The XX1 BB30 crank's axle measures ~89mm without the bearing preloader. 68mm BB faced at local shop, measures at 68.3mm.

I bought external BB30 BSA threaded bottom bracket bearing cups from Enduro. Each bearing cup measures ~10mm, so with BB and cups, total came to ~88mm. I installed the NDS arm minus the preload adjuster. Sure enough, the crank interface barely clears the bearing cup. I installed the DS without any spacers and torqued it to spec. Chainline measures at 49mm per SRAM. The crank spins freely without any drag or play. I've been running this setup without any issues.