I managed to get my distributor issue figured out.
Since my ebay purchase turned out to be wrong for this vehicle, I ended up putting the ebay distributor on my El Camino to great success. The old distributor was just clapped out and just had terrible shaft slop. The mechanical advance weights had also ate about a third of a way into the pivot shafts.
At this point, I figured that this distributor's hard parts were not worth rebuilding so I took the Buick distributor apart and started comparing internals. They appear to be a lot more alike than different, and nothing that screams that the parts cannot be interchanged. The big differences are that the Buick distributor has no mechanical or vacuum advance, while the Chevrolet distributor has both. The Buick distributor has an extra notch out of the housing for the ESC harness that the Chevrolet one does not.
I removed the roll pin from both distributor gears and pulled the shafts out to compare. The reluctor gear on the Buick distributor(short) is fixed to the plate while the Chevy distributor(long) has a mechanical advance plate that floats on the shaft with the reluctor gear attached. This is tied to the top plate by the mechanical advance springs.
The top plate has a pressed in bushing that fixes it to the shaft. There is probably a good way to remove it, but I couldn't figure it out, so I beat the shaft with a hammer until I drove the plate off. I did the same with both distributors and switched the V6 reluctor and top plate (fortunately one piece) onto the Chevy shaft. At this point, it is in the correct place but the bushing is trashed. I decided that I will just tack weld the top plate to the shaft, since I will not be doing this again one way or the other.
With the shaft converted, I took the base and removed the guts and switched them between the two. Everything more or less bolts in, except the 7 pin module has a slightly different bolt pattern than the 4 pin that came out. Not really a big deal, but I had to cut the locating pins off of the bottom of the module and bolt to one of the existing holes. The other holes hangs in free space, but I need to drill a new hole and install a longer bolt to anchor it properly.
Under the silver cover plate is the pickup coil and a mating piece to the reluctor. Basically switch the middle stuff from the Buick distributor to the Chevy one. Once that is all set in the ESC harness is formed to the housing, and pretty well shows exactly where to cut into the Chevy housing to let the bulkhead connector set in. A little dremel work later and it fits like a glove. Reattach the ground wire to the stud for the vacuum advance, and slide the shaft back in. I reinstalled the flat washer and the anti-rotation washer between the base of the shaft and the distributor gear, but added the wavy washer that was used in the Buick distributor to make up for all the shaft play that the Chevy distributor originally had.
Hopefully it works, but I have never broke one of these distributors down before today so hopefully all my guesswork pays off.
