It looks like it could be cylinders not honed properly or wrong oil rings.
Have you checked the compression? Did the guy who put it together check the
ring gap in the cylinders when installing?

Have you pulled the plugs to see if it's all cylinders or just one that's
oil fouled?

What work did you have done on the block?

Unless someone has any better ideas, you may have to pull the engine and
inspect it to see.

If a shop did the work I would make them fix it...

In my experience smoke on acceleration are valve guides leaking, smoke on
deceleration (high vacuum) are rings.  Perhaps the valve seals are the
culprit.  I'd check the spark plugs maybe you can figure out which
cylinder(s) are the problem.  I'd take it back to the shop and say "FIX IT"!
You seem to have broken it in correctly...

Butcher
'96 LT-4 CE

Eliminate the little things first, check the PCV valve in the back of
the carb and it's vacuum path. Run a vacuum test and pay close
attention to how the gage reacts, cheap gage and instructions should
come with it. With the oil fill cap off do you feel a pulse of air
when it is idling? If nothing shows up there it's time to quit
guessing and pull the plugs and take a good look at their condition.
While they are out, run a compression check. If you don't get a
noticeable blue puff of smoke at startup after it has sat for awhile
it's not the valve seals. If you find one plug that looks like the
culprit you could change those seals and see if it makes a difference,
my guess is it may not.

By the way I did not say 30 weight non-detergent oil and I wouldn't be
running that weight in it in the first place for break in after a
rebuild, maybe even 20. Please note that this is no longer recommended
for today's hotter running engines but worked on the engines of the
early '70s and older. Information came from an old sprint car engine
builder and it actually requires the rings to break the lubricating
film between them and the cylinder wall.  Your engine builder may not
be relating to the age of your engine as well, he must be working on
more modern engines that require a heavier weight detergent oil at
startup.

Interesting read - http://www.tcmlink.com/visitors/carenfeed/brkin.pdf

Good luck.....

You have a problem....
Could be valve seals, on DECELERATION you will see smoke
from the vacuum created in the cylinder sucking oil past the seals.
But it doesn't sound like it from your description.

My first suspicions are that the machine shop not getting a good pattern
on the
cylinder walls during the honing process. Too smooth of stones or bad
crosshatch pattern or the combo of the two. And possibly boring to wrong
specs. (oversize). But you should hear some piston slap when it's cold
if the cylinders are too big for the piston/ring package.

Or the builder broke some rings, or didn't stagger the ring gaps. Or
left off one
or both of the thin oil ring support rings. Or hand filed the ring gaps
OUTRAGEOUSLY
too large. With today's metallurgy you shouldn't be having any ring
seating issues at all.

Finger pointing time with no resolution.