Wade:
Nice build. Looks like you put some thought into the design. If I may, two areas I would suggest you address. I don't know what the floor plates back up to on the rear tunnel but that 14 gage sheet metal floor is like a beer can when it come sheer force. If you can use a piece of .250 plate to tie the tunnel beck to the frame connectors and then bend a U shape plate to fit in the tunnel. That will give the inside cage plates something to get support from. The second benefit is the .250 U plate in the tunnel can act as the top part of your rear drive shaft loop.
People on this forum are probably sick of me saying this but in 2011 I had driveshaft break and instantly is cut through the factory floor. The shaft was in the car with me and chopped the sheet metal up good. Had my seat been back another 1" the shaft would have been chopping on my seat and hip.
Lastly, if you are going to take it to the salt, when your done welding, look into some Por15
http://www.por15.com/POR-15-Rust-Preventive-Paint_c_11.html Its about $100 to paint the underside and the interior floor but it will help that unibody last. Your car's first life as an SCCA mule is nothing compared to the beating it will take from the salt.
Keep up the good work.
BR