FWIW, I have found that with two-piece shafts it is sometimes difficult to get the distributor reinstalled properly on the oil pump drive gear if the lower shaft is attached to the distributor. So generally I separate the lower shaft from the distributor, install the lower shaft into the oil pump drive gear using a screwdriver to turn the shaft until it slides into place, and then install the distributor on top of the lower shaft. I've found that on some aftermarket lower shafts the offsets are just enough out of whack that I have to turn a couple thou off the shaft ends to get them to slip into the couplings. So V4F, if you have a new lower shaft, this may be the issue - the shaft won't slide into the offset connector.
As long as the engine has not been turned over after removal of the distributor, it is really not possible to install the distributor 180 degrees out. That's why the oil pump drive gear, the distributor lower shaft and the distributor top shaft have offset engagement. Timing to 180 degrees off can only happen if the crankshaft has been turned one full revolution so that the #1 cylinder is at TDC of the exhaust stroke, and then the distributor is retimed to the #1 cylinder thinking it's at the top of the compression stroke. But the depression in the timing gear will not line up with the hole in the timing cover at that point.