Stumbling Slant --what is left??

The story so far: 68 Dart with a slant. I noted that the distributor shaft was getting a bit wobbly when I last tried to change the points, so I tried rebushing the distributor I had, failed at that, and replaced the distributor with a used one I had. I might have had a bit of hesitation before this, but now I had real stumble--at idle, under load, all the time. OK, so I replaced the distributor (cap, wires, plugs, points, etc) with a new one. No change. Changed the coil. No change. Looked at the timing chain. It was loose, but not bad. But I replace the chain and cam gear anyway. No change. Checked the compression--all six between 90 and 118 PSI. Checked the valve lash, tweaked them a bit (none more than 1/8 - 1/4 turn) No change. Took a 2 liter bottle of gas and fed the carb with nice clean gravity-fed fuel. No change. Installed a rebuilt carb. No change. Wife suggested exhaust, so I just now unbolted the exhaust from the manifold. No change. Early on had installed new PCV, fuel filter to no avail. Interesting: the new plugs with barely 30 minutes of idling on them are showing carbon black. The old plugs that came out were fairly black, too.

Any ideas? Please let me know.

Thanks.
Bill