You can buy adaptor bars from a seat manufacturer (mine were from cobra), these bolt directly to the standard subframes which you then bolt the sidemounts to.....no cutting or fabricating needed and you can adjust the seat as standard.
The seats do sit a little higher though so maybe not the best if you want them on the floor lol.
Mine are for sale out of my rst, here they are out the car so you can get the idea of how the bars fit the standard subframe: