My current car has seat runners sat on aluminium box square which is then bolted to the floor on the car. I am looking to change the runners and of course there are different lengths. So can I bolt the runners to the floor of the car, or do I need some form of spreader plate. I have some very large spread washers I could use, but do I need to put some plate down to

For IVA yes load spreading washers
If you've passed IVA still fit load spreaders or seat can flex
Even with load spreading washers if your floor is thin or ally cross bracing is prefered to stop the seat loads pulling the floor, it's not a nice feeling having the seat move with e G force

I bolted my runners directly to the floor but put some steel strip underneath the car just to act as a spreader plate.

Cheers gents, many thanks for the replies.
Cheers
Craig