[Harp-L] Key of song from Guitar cords

[Toad <soundguyaudition@xxxxxxxxx>]

for starters I have -0- music therory, 
my Question is, if the guitarist calls out cords like C7 B13  Em7/9 , 
1. is all I need to know is that the song is in the key of C, then pick up the appropreate harp and play in the correct posisition for that harp to be in C? 
2. do the numbers dictate what position I should play in?
3. or are the numbers irrelivent to harp players?
 
Sorry if this is not explained well enough to get my question clear.
 
thanks
Todd