TECH TIP : Sometimes the side of the seat is not square. So, the seat brackets will be splayed outward. If everything is just right the seat will just drop right into the car and the bolts will go in EASILY. So, now as part of my build process one of the last things I do is lay the seat on it's side on a moving quilt on a table top and measure the distance between the bolt notches. I have found that the 18.5" seat is the worst. I assume it's because the brackets don't use spacers with this setup. The brackets are usually splayed outward and that dimension is over by about as much as 3/4" ! It should be 16-1/2". So I carefully bend the brackets by pulling downward to get the 16-1/2" between the notches. I bend each side equally as I can. Be careful and don't over do it.
Also on the fit. I am 5'9" tall and 185 pounds. I sit in a 17" and I feel right at home. If I sit in a 18.5" it's like OMG way bigger. I don't like it at all. So it's quite a bit bigger on the fit. They also make a 15 and a 16 and a 20 inch wide seat. These brackets would not work with the 16 or the 20" but likely would work with a 15" if you turn them around. Best thing you could do is if you KNOW someone that has this seat is go sit in their car. If you are around me just ask. If I am not around my car at the time then just go for it. I won't mind.