Good! Putting a little Marvel Mystery oil in and turning carefully sounds like a good idea. As long as the oil flows and feels slippery, should do the trick.

Consider getting temperature on your side by using electric dipstick heaters and magnet attached block heaters.

Alluminum pistons expand at a different rate than cast iron block walls.

I personally would not try it first,
but there is a CRC Freeze-Off spraycan product (78254 05002 barcode)
that could be sprayed with a long tube through the sparkplug holes
directly on the piston crown to cause it to contract
and move the rings enough to potentially free up solid corrosion bindings.
Two or more cold/rewarm cycles might be needed.

Video boroscopes have become relatively cheap.
It might pay to use one with a 9 to 11 mm camera head
to look through sparkplug holes at the piston crown
while someone gently rocks the crank with your breaker bar.
You will see free pistons flex a bit inside rings that are not stuck
but problem cylinders will move little at all.

Remove the distributor cap
and watch for slight rotor movement too.

Good Luck.

Tap each rocker arm with a mallet to make sure valves are not stuck in their seats or better yet remove the shaft and tap the springs. I had a 383 once that sat inside for several years and was mildly stuck from sticking valves. Engine was fine, put many miles on it after

A friend had a stuck 273 in the car from sitting, got it free with ATF, got it running and it was solid smoke for 30 minutes, then it was all clear and ran fine.