1)Well the rotors need to be drilled for the bigger truck studs. The impala uses the 9/16 lugs and the c1500 uses a 14mm. I will post pic later of what needs to be done.
2) you can make the bracket a bolt on (I have done ths one time when I did not have a welder.i will add photo of this set up...
Yes it will work with 15's . You just have to grind on the caliper a little. The rest is just getting the brake line to fit. Drill the rotors for the 14mm lug studs.
bracket
http://www.smileysracing.com/shopping/productDetails.aspx?i=13569&c=0
caliper...