There is a spec for the posistion of the boltholes in the floor, surely, to determine the position of the subframe in relation to the front end.
Did you get a actual replacement subframe, or was it used or repaired? If, like mine at the moment, it has new links on one side and not the other, it may drift until you get them replaced -- under load the torque link will allow the toe on the rear wheel to change. Mine drags just like yours, a bit less -- if I let go of the wheel, it will slowly drift right. Didn't do this before the change of links in the rear, so I know without doubt this is the issue, and I'm waiting until I get the $$$ to buy a whole set of links and subframe mounts, then will drop the subframe and do the whole shebang.
If you have bad subframe mounts, the rear subframe may be moving under the car, not much, but enough to steer funny.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!