This would be a longshot on a rebuilt engine, but could it be possible that your PCV valve is stuck open? I had one stick on me many years ago and the symptoms are identical to what you are describing.
55 Cameo 396 TH 400 with overdrive 65 Caliente conv 67 BB Caliente conv 67 Mercury 1 Ton 76 GMC 3/4 ton 454ci 84 Buick Turbo Regal
