Usually, smoking under a load is a sign of worn rings. I guess you have realised this as you have done a compression check and checked the breather.
A common problem for these is for the valve guide oil seals to wear. However, symptons of them is smoking on start up (after being left, say overnight) and decelerating from a relatively high speed run. this doesn't match your description.
Would you be able to describe the smoke as blue (oil), black (overfuelling) or white (water)?
Out of interest, what were your compressions? And did you do a dry and wet test?