the #8 cylinder condition could mean low compression/valves worn/burn't. could also be bad plug wire. using a meter measure all plug wire resistance and compare. this should be 10k to 15,ooo ohm area.
the oil used should be 10-30wt. 5-30wt is good if your going into below 32 deg temps... ,,,, like where I live, arizona stay with the 10-30wt.
the valve seals may help..the real problem with smoke on start up is the valve GUIDES ...this would require removal of the heads, then a machine shop to correct this manufacturing defect that all these 305cu 80's engines have.
oil changes must be done when the engine is hot. if warm or cold the sludge is not going to come out. remove the valve covers and using a coat hanger / stiff wire make sure all the head drain back holes are clear.
most times the drain back holes at the back of the engine are restricted with sludge/carbon/ or even manufacturing dirt/gasket material .
this may be why it smokes and why #8 has a build up on plug.