Flat floor/stock subframe camaro?
 

As I'm going through the rust on my camaro(roof, quarters, trunk, rockers, rear rails, and parts of the floor) I'm tempted to do a flat floor in the car for a few reasons.
1) I just rebuilt a sweet bead roller
2) I'm going to need to make tunnel/firewall mods
3) easier seat mounting
4) easier pedal mounting
The problem is I'm going to retain the stock subframe/mounts, is there a way to keep these with a flat floor? Do I have to have braces from the cage to the points? And what gauge steel should a flat floor be made of?