E21: Seat Fitment Guide:

Swapping seats round in a BMW E21 3-series is a nightmare, thanks to the floor-mounts being two different heights. To fit level-based seats from another car requires chopping out the welded floor-mounts and once this is done there is no going back.

1. Check if your floor-mounts have been chopped:

Check the outer floor-mounts on either side, spot-welded in place just inside the door-sill. They should look the one in the left photo below.

If they don't, and they look more like the right photo [with or without the wood], then you have chopped mounts and seats not originally from an E21. You also won't be going back to E21 seats very easily, so to preserve the style and make the ride a lot better, throw in seats from a newer BMW.

2.1 Fitting stock BMW seats from E30, E36 etc. :

This makes sense. E21 seats are sprung and uncomfortable and were dropped in almost every succeeding model. Other BMW seats are easier to come by and cheaper. E30 seats preserve the look almost identically and are far superior in construct. They also fit the inner seat mounts on the E21, the ones against the trans.-tunnel, so only the only outer ones need adapting. I am sure this is the case with seats from other 80s BMWs, possibly the E36 and E46 as I've seen it done, but need confirmation.
Modifying the outer floor-mounts, the ones against the door sills, is impossible as they just sit that much higher than the inner ones, the E21 of course having a crazy odd-height sub-frame design, so they need cutting out. Putting in new ones is pretty straight forward though, you can fabricate them with steel or alloy C-channel, wooden blocks, improvised stilts - these are all popular methods. Tilt-forward seats don't tend to be very strong anyway, so the new mounts don't have to be rock solid as long as they're bolted or welded securely to the floor-pan. Mine used the stock inner floor-mounts and a wooden block on the outer side to get the height - this presented no problem to the UK MOT (roadworthiness-test).

Check out my guide here: http://e21dub.blogspot.co.uk/2013/09/e30-sports-seats-fitted.html

Or check out HSVTurbo's guide HERE [where I got my idea] in THIS THREAD on bimmerforums.

2.2 Fitting rigid Racing bucket-seats:

If you're building a track car then it's obviously best to keep things as stiff and secure as possible. The neatest and most weight-saving method is to cut some steel cross-beams to fit and weld them across the floor-pan, from the trans.-tunnel to the door-sill. This could be done with or without removing the outer floor-mount, but you will probably have to do this to get the seat low enough. Two cross-beams under each seat would probably be best, but it is most often done with just reasonably thick one.

More info. HERE on bimmerforums.

2.3 Fitting non-BMW stock seats (VW, Ford Recaro etc.) :

To get the right height and a central-position with stock seats from non-BMWs there is a lot of modification to the mounts - these seats will always have the bolt-holes in the wrong place. The tidiest way would probably be to weld in rigid cross-beams like in the above guide [2.2], otherwise you will need to fabricate custom mounts. [I used a combination of wood-block and steel cross-beam to fit Ford RS Recaros to mine in THIS POST].

Check out my guide here: http://e21dub.blogspot.co.uk/2013/08/recaro-seat-base-adapter-finally-solved.html

3. Re-fitting stock E21 seats:

I have never seen anyone online do this. If you have level floor-mounts then I see no reason to switch back to E21 seats. All but the Recaros are uncomfortable, sprung nightmares. They give a truly bouncy and uncertain ride - only the most restoration mad would want it. The Recaros are super-rare and overpriced to fit the E21s mounts. E30 Recaros are a fraction of the price, have level mounts and are better anyway. If you are so desperate to restore your E21 dutifully, then I would consider it having cut floor-mounts a deal-breaker and to find one with stock seats, because the car will never be the same again.

There are 3 ways I see to achieve refitting stock seats:

1. Build custom outer mounts using a combination of the methods above, building them back up to the original and rather tall height for the stock seat-frames. [].

2. Locate a pair of original outer floor-mounts that have been cut out of another E21 and refit them to yours. These would first need to have been cut out carefully from the donor car and preserved by someone. Whether they would advertise them for sale is another matter, but asking on the forums can't hurt. Otherwise, you'll need to find someone with a scrap / breaking E21 and ask to neatly cut out its floor-mounts for a few quid. Either way, obtaining a useable pair is going to be tough.
Once you do though, they would just need to be measured up to where yours were cut off and welded into place. How neat a job you do will determine whether they fit or if it's unnoticeable.

3. Fabricate new outer mounts from the ground up out of steel and weld those in. I will endeavour to make a note of the dimensions of my cut one before passing it on and do a quick CAD design to fab-up a simple one.

**Also, I still have the passenger-side (N/S, Euro/US Driver-side) floor-mount [above], so if anyone wants it to try and weld back in message me! [Sadly, I cut the driver-side one out more severely and this is probably going to be the case if yours are chopped as it gives the wood or whatever more to grab on to.]