Hei Ricola!
Beautiful car! I am building my 1303 with Porsche Boxster seats. Sad I haven't seen your way of mounting them before now... Your mounts looks really clean and easy to build while mine are bigger, heavier and takes more place.
Could you please give some more pics of how it was done with some "prescription"? How did you find the right positions for rear bolts and whats the hight of you sitting position.
Boxster seats are so low that I needed to build pretty high mounting bracket. But I also struggle with body on pan. Is it possible to make your type mounts with full body car?
Hope you can help with some experience