seat supports

[Badger]

Personally I'd be tempted to leave the seat mounts until quite late in the build - just clean off the paint around the area you need the mounts when you want to fit them. The problem is (as MikeB mentioned) that there is no standard fitment, so it'll depend on what you get.
Your alternative is to get your seats now and make an educated guess at your driving position (easier if you have the column). This also allows you to make "Brrrm brrrrm" noises and pretend to drive it. Not that I've ever done that mind