Been wrenching SBC's (since '66) almost as long as SBC's have existed.
And on of the first epiphanies I experienced was how to re-set
the distributors.
As stated above, the source of the problem, is that as the dizzy is
lifted out, the shaft rotates as it disengages first from the oil pump
drive shaft, then the cam drive gear. For this reason, marking the
INITIAL position of the rotor is not nearly as important as marking the
FINAL position, which unfortunately is neither as easy, or intuitively
obvious. Regardless, the likelihood of getting the shaft to rotate
back down into the exact position it left the oil pump is slim-to-none.
However, think about the problem a bit more. Obviously, the end
goal is to get the ign. timing properly set. The oil pump shaft will
come along for the ride.
SO...
Insert the dizzy, getting the rotor to finish as close as possible to TDC,
when aligned with the #1 contactor in the dist. cap.
(And remember, the crankshaft passes TDC TWICE during
a combustion cycle, so make sure its on the ign. portion.
Easy enough to accomplish if you have the rotor at #1,
and the crank at TDC, when you start disassembly.)
With asistance, or a remote starter switch, turn the engine over,
while holding the dizzy body. At some point the dizzy shaft will
align w/ the oil pump, & drop down into place -- works every time.

FINAL adjustment is made w/ timing light, according to specific
enging spec's. i.e. for EFI, different setting, while ECM is config'd
for ign. timing.
Good luck.