How to install bucket seats?

If you used 2x4 you would still need longer studs and you would want to make sure to use pressure treated wood. And make sure the wood is being compressed by the hardware, do NOT attach the seat to the wood and then attach the wood to the floor. There isn't any reason good sound wood can't be used as the spacer.

I used a 2x2 for 10 years in my valiant with daytona leather seats and the wood was still solid and secure when I pulled the seats last winter.