I was going to post a link to a seat adapter from classic industries but reading it closer it seems that they are to replace original seats to match up to the procar seats, i dont think it matches the floor pattern... so above post is about it i guess.
I have been thinking about using 7/8 slotted unistrut so I can level and bolt it to the floor. Then I can adjust the seat side to side wherever it fits the best. I currently have Cavalier seats that were not quite centered with the steering wheel. Then when I change out the seats, I don't have to make new holes in the floor and carpet.
Here is a photo of the brackets that I made. I welded grade 8 bolts to the brackets and used them as studs. The seats and brackets came from a 1976 Ventura.
I would recommend trying the fit if possible before making it permanent.
Also another thing worth mentioning is that if you decide to make your own bracket, make sure that both seat tracks are in the same position either all way back or all way forward. This will assure that the seat is mounted square. My seats sit a little bit high up for my taste but I am 6'4". I plan to cut and re-weld next winter and hopefully gain a half inch or so. I think my bracket was 3/4".
Hope this helps.