Nick my bet is that you need to hook up the power seat cables and play with them until the holes line up. When i was trying to get the quarter out of the track I had to play with moving the rails around otherwise the holes wouldn't line up. And as for stacking washers, just make a metel wedge. Like get a little thick piece of steel and grind it into a wedge.

Btw does the back seat work fine also?

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1981 300SD 512k OM603