Under Floor Supports?

[Kartch1]

Hello all,
I'm building a 56 Chevy and am replacing a large section of the floor and firewall. This thread shows what I'm replacing.

I'm planning to use C-channel to replace the factory brace that the seat mounts to. I'm going to match the seat mounting holes by drilling and welding in nuts before welding the brace to the floor.

My question is this.....

I would like to weld some bracing under the floor that run from front to back to add some rigidity. What do you suggest I use? TIA.

Here is a quick shot of a part of the floor replacement.

The area with the X's is the factory cross brace that will be changed out for C-channel. I'm looking for something to run underneath the floor perpendicular to that brace.