If the symptoms have happened all of a sudden my moneys on a Cracked / broken piston ring land......
its happened to me a few times now
On both occassions Pistons appear to be fine even when removed from the Block at first glance, but with a bit of fiddling you can remove the piece of piston between the rings themselves, generally under boost they are compressed and oil does not seep thru hence no smoke under load, however once you let off you leave a nice stream of smoke behind you! its normally caused by Detonation, lean out and can happen extremely quickly
Is you intake system full of oil also?, or your catch can (if Fitted) filling with oil?.
Maybe your car since rebuild has got hot, maybe within one cylinder, blown the gasket, and cracked a piston....?
Leak down test best way to check for Piston ring land.