Any question answered...

Headset spacers aren't wide enough, outboard bb spacers work though

Internal 1.370" (34.8mm, 35.0mm is better to give a bit of clearance) external needs to be no more than 39.0mm to clear most chains when using a 12t sprocket

40mm PVC waste pipe will do at a pinch, will only work with sprockets >13t, 6 bar or 10 bar rated will work, 16 bar won't because of the excessive wall thickness.

Thanks guys...