1983-300SD-150K miles
While doing 100mph , temp up from normal 80 to 90 then back two times
At same time oil pressure from 3 flipped down to 2 up to 3 two times
Smoke poured out of vents
Slight power loss
All gauges back to normal
Slowly drove the 1/4 mile to off-ramp , wub-wub-wub sound down low into Stuckey's
Shut car off
Poured in 3 qts of oil , checked level - LOW
Added 6 qts (3+6 = 9 !! )
Let car cool for 3 hours , cranked
Heard a taptaptap on front cylinder at top
Shut off , trailered it home
All valve clearances are to factory specs
Car runs fine - have driven it ---- the faster you go , the less you hear the tap
Turbo sometimes intermittently
Now has begun to smoke and have low power ---so I parked it
Any ideas ??? OR ......... do I ...... rebuild / replace ? / buy another car ?

NOT good. You have probably damaged your engine by running low on oil. Don't know how it escaped, but apparently it did saying as you added that many qts.

It dont look goof for your engine IMHO, drive it till it drops.