Just my thoughts on this seat swap. I bought a pair of F30 "base model" seats with their pigtails:
Swapped over the Toyota seatbelt assembly, using a bolt, steel sleeve and bronze bushing to adjust for sizing and spacing. Just stuff I had available in the shop bins, so no clue on sizing.
De-pinned the BMW seatbelt lines and put the plug back in place so everything will still line up and work right with the connector.
Fitting the seats into the 4runner was more frustrating that difficult. The seats are about 1/2" narrower than the 4runner at the rails. But the inside 2 and the front outside all bolted up, thought they clock themselves a little bit. I had to start the bolts and tighten them slowly as they all pulled themselves into place. The outside rear though, needed a new hole:
because it was quite a ways out:
Sadly, this mount doesn't bolt through one of the reinforced areas designed for bolting the seat down. I may revisit this in the future, when it's not so cold and wet outside.
Used 1/4x20 flange bolts, washers on both sides and a few different nuts. Basically, ran a washer to the flange head and then a nut up against that. This fit into the space in the BMW seat so it wouldn't slop around, then through the brackets (Toyota rivets were 1/4", hence my bolt choice) with a washer and lock nut on the bottom.
Still have all of the electronics to deal with. My seats seem to time-out in less than a minute so making adjustments is a pita of stop, connect wires, make adjustment, disconnect wires and drive till I find the need to make another adjustment.
There is a Whole lot more seat travel, especially in the back direction:
even at 6'4", I'm well away from the back stop. Have to get my 5'4" wife into the drivers seat to make sure there's enough Forward range....
They do fit the space well though: