HELP...grass problems!

Ill try to make this as short as possible. I live in garner, just
inside the johnston county line. I have the most hardest, roughest,
crapiest clay soil known to this area. I have 2 problems that I would
like to address. Problem 1, the contractor used a contractors mix on
my lawn about 4 years ago when I bought the place and it had a
abundance of bahia grass seed mixed in. PLEASE help me make this go
away with ripping up my whole yard and starting over and heres why:
Ive got about 40% of my lawn covered in centipede that i want to keep
since I do not have a whole lot of shade. I have alot of bare spots
and I am thinking of overseeding this fall with Southern Gold. Has
anyone ever used this seed? I want something to cover up the weeds and
bare spots while my centipede continues to take over over the years. I
will only concentrate on fertilizing and taking care of the centipede
and the Southern gold will only be there for looks for now. So, my 2
questions are: 1. How to get rid of bahia grass and 2. Does anyone
have experience with Southern gold. Thanks for any advice in advance!