Any pics of aftermarket buckets?

Not sure if it helps you any, but I am currently putting in the sport seats from a 2003 Audi A6 in my '72. I would imagine a 5 series, especially something around the e34 chassis, to have similar sizing. The Audi front and rear seats are almost the perfect size, probably won't have them completely installed until the rest of the car is finished though.

Not sure if that helps even a little, but I'd imagine you could get them to fit!