Well all's well that ends well, I guess... but...
I decided to take her out for a drive last night and quickly realized the seat belts didn't fit in the seat receptacles. The SRS light was also on. Long story short, the donor car had been in an airbag accident and when that happens, the seat belt receptacles pre-tension and become unusable. This morning I pulled the seats back out, swapped on the receptacles from the old e46 seats (I had to splice one wire, but it wasn't much of a PITA) and put it back in. I have the BMWHat app and Mycarly OBD programmer so I was able to reset the SRS light and now all truly is well.
The seat belts needing replacement wasn't mentioned on the seat listing. Had I know, I probably would have gone with these:
e63 M6 seats. They are also a drop in, but this ad disclosed the need for new seatbelts.