I have Kirkeys in my Birkin as I have had in previous track cars. I ordered the 17" road race seats and, while the bases fit perfectly, the shoulder supports were too wide to clear the rear fender arches. This was with them sitting in the SS3XS, the 100mm wider Birkin. I ended up cutting them down a bit and having the covers modified as well. Were I to do it over, I would order the Kirkey drag seats in the same size. They are made with the same inclination but don't have the shoulder supports. The covers that come with them should fit fine. Remember, with Kirkeys. all of the padding resides in the seat cover.