Hi Ricardo
To be honest, the Smart Car seats were a bit of a pain to fit. It's not as straightforward as you might think.
I'm 6'2" so need the seats quite a long way back.
If you're shorter, then fitting is easier.
The main difficulty is the right side of the tunnel where the hot air comes through from the engine.
That narrows the car quite a bit and required some serious bashing with a big hammer to give clearance for the seat runner.
I also had to cut away some of the seat metalwork to give clearance. (you can see this on the photo)
The 126 seat runners were in the way so I cut them off with an angle grinder.
If you get some Smart Car seats, make sure you get the subframes they bolt to and the seat belt stalks that are bolted to the seats.
You can they concentrate on fitting the subframes to the car & then simply bolt the seats to the subframes.
The seats end up very close to the door frame, so before you do the final fit of the subframe, put the seats on the subframes to make sure all is OK.
The odd thing about the seats is that although they look the same, they are different left/right. The subframes are completely different and the seats themselves will only fit on the correct subframe, you can't switch them over.
I fitted the right side seat on the left side of the car & the left side seat on the right side.
You'll need to remove the "ears" near the headrests the seatbelts pass through. I cut them off with an angle grinder.
You can bin the 126 seatbelt stalks as the Smart Car's bolt to the seat itself and the 126 belt buckle fits perfectly.
Also, on right hand drive Smart cars, only the right hand seat reclines. The left seat will tip forward but not recline.
Having said all that, I'm well pleased with the result.
They're probably not the comfiest of seats, but they're a heck of an improvement over the originals and look much better too.
I guess they should be safer as the headrest is much taller than the 126's.
Good luck