How much of a difference is 13" brakes over 11". My engine will have less then 350 hp. Will 11" brakes work.  

yes..... they are designed to stop the car as  gm designed it..   if you bump the power , weight, speed of the car when brakes are applied...  (like going 140 miles an hour down the drag strip)   but that point is moot as most people put narrow tires up front and remove any chance of stopping the car or steering it at high speed using the front wheels..

so put the big rotors and calipers on the rear..  and a parachute..

you might think if you have the factory 68 4 piston calipers to get a reman caliper.. if they have not already been installed... and at least one of the camaro parts houses have replacement  rotors for the 4 piston calipers...  the 69 and up rotors will not work in most cases....

also... i have not checked the aftermarket  race pad companys.. to see if anything other than organic pads are available...

the 69 camaro, firebird,, and 69 to 74 nova, omega, ventura, apollo spindles, rotors and single piston calipers bolt right on...  and give you  a much wider selection of brake pad materials..    organic, semi metallic, carbon metallic, ceramic, kevlar, carbon, and what ever else is out there for a D52 pad...