You might give S&W a call, talk to Don Schol. If you are not going to build a complete chassis I would definatley leave the firewall in. As far as the floor goes you can take it out, but you need to run your frame rail all the way to where the torsion bars mount and put some X bracing in the floor from the frame rails to the rockers and also put in rocker bars. It will make it a whole lot easier to fabricate,measure and weld. It is very hard to get 100% welds with all the floor in the way. Plus you can mount your belts and seat to somthing other than sheet metal. If you can get ahold of a SFI chassis book that will help you a bunch with what you actually need to make the car safe and certafiable. It will suck if you put a lot of time and effort in your chassis and it won't certify. You might check with some local chassis shops, you might be supprised at how reasonable they are to do a back half.
Good luck on what ever you decide!