How close is too close?

it might catch when the cranks flex under distress (hill climbing etc.)

miche BB's are adjustable by a few mill right? i'd push out those cranks or use a longer spindle and put the chainring on the inside of the spiders.

whats on the back that's giving you 40mm?

It's a Sovos hub with a Dura-Ace sprocket one side and some unknown make the other. If I pushed the chainset out further I would have run out of thread for the sprocket after I'd fitted a spacer behind it.