A guy on chargerforums put 275/40R20s all around and has not had rubbing problems. I haven't seen any posts of a 300C SRT8 using that size though.
http://www.chargerforums.com/forums/...highlight=tire
For the intake, you might want to try a ram air. Here's one I did with the help of some other posts I found around here:
http://www.300cforums.com/forums/srt...-air-srt8.html
I don't see how you can get any more cold air than this.