Blowby / Valve Adjust

My 79 SD has unknown mileage and no service records. It runs well and is strong enough to use comfortably as a daily driver. I have replaced the fuel return lines, the bad glow plug, the fuel shut-off valve, changed the oil and tranny fluid. I considered next doing a valve adjustment, but there is quite a bit of blowby evident when the oil cap is off while running. Will a valve adjustment do much good at this point? Should I set my sights instead on a top end rebuild?

Wouldn't the blowby be a common result of the valve adjustment being neglected? My theory is that the valves being slightly out of position at the time of explosion would lead to excessive wear.