I have your EXACT symptoms with my rebuilt l28. Have an f54 block, n47 head, fresh rings, bearings gaskets. Honed the block , checked ring gap, everything is speced out and should be perfect. But no...it smokes like a mofo. Tried rebuilding the carbs, no help. Tried pulling the head and replacing the head gasket and putting on a whole new head that was on my other z (working perfect) didnt help. Put in different 3 quarts of 40w oil and 1.5 quarts of lucas oil stabilizer, still smokes. I havent run it much other than in the garage at idle, and smoke quick revs here and there. Only thing i can think of is to do like others have said, and to take it out and run it hard to break it in. BUT, my car is all apart ready to get body work and paint done. So until that stuff is finished it will sit as is. Let me know if taking it out and running the snot out of it helps.