Don, your explanation doesn't make sense to me unless you've got bad bearings, a bent shaft or something else that's putting resistance in the rotation of the distributor shaft relative to the distributor housing. If the housing, bearings, and shaft are in good working order, the shaft spins in the housing with very little resistance, and thus the shaft puts virtually zero rotational force on the housing. There's just nothing there to make the housing "creep" on the block, and it doesn't matter how much vibration it sees, which on your engine should again be virtually zero. It just doesn't compute that your distributor housing is moving in the block because of gear forces. There is just no way for those forces to affect the housing, seems to me, unless you've got bigger problems with that distributor.
What I did experience was that on my SBF (yes, Ford, and I know your's is Chevy) my distributor housing was quite tight in the bore on the intake and I failed to note that when I stabbed the distributor the housing was slightly raised, not firmly seated against the intake surface. When I set my timing to the base 10BTDC it was precise, UNTIL I tightened the hold down, which pushed the distributor down about 1/16" to 3/32" and advanced my timing three or four degrees due to the helical cut of the gears. I felt really, really foolish when I realized the source of my timing problems.