Valve stem seals are gone, not doing their intended job.
You might be able to put GM's spring hood/cup on the valve spring along with replacing the stem seal and installing stem cup seals on the valve stem/guide.
This can be done without removing heads.
Or, remove heads and have headwork done, exchange heads for rebuilt ones, or buy new ones and re-install.
But then, the old wives' tale, "You build the top end, the bottom end will fall out" I personally never experienced this, but
It depends on how deep your pockets are.
Others here may have some good suggestions, on a fix, the question is, how comsumption is there and is the blue smoke really that bad?