If the original body cross members are in place, you can just make filler pieces and either bolt or weld them in place - the originals were riveted down. You can also just skin over those cross members with individual pieces too. Then the trunk/or rumble - if it's a trunk, a flat floor the same as the pass. compartment will work. If it's going to be a useable rumble seat, then you have to make up an angled riser to bolt the seat to. I wasn't happy with what Brookville Roadster provided though they are replica's of the originals so I made up replacement filler pieces with a reinforcing grid between a top and bottom panel. That grid is used for seat bolting as well as the seat belt anchors.
I 'lost' my floor board photos