Well the fifth string is the one you start from with your thumb when its open G. Cover five on the fifth and the seventh fret for c and d and use the sixth string as a deep bass.the two finger am7 shape will also give you C, and fretting the secon and third string at the first and second fretts will give you an acceptable d7 - sort of sound.

Experiment with sliding thes shapesup the third fret - two shapes forward

Heres me messing about with the blues form in that tuning

http://www.youtube.com/watch?v=oN-gTsyQ2zs&feature=plcp