Is it a stock C4 or a rebuilt or purpose built racing one? For purpose built ones, they make C4s that'll handle 1000HP.
You can have a factory C4 rebuilt with your HP number in mind. Local shops around me charge about $900, including removing and reinstalling the transmission to the vehicle. You can go on up from there.
Front brake conversion should be simple enough with either factory parts or with an aftermarket kit. The C4 can easily handle a 350 HP engine, you will probably have to rebuild it with some good parts, put a shift kit in it, and a converter to match the cam you are using. A 73 Grande with a 302 will probably have an 8" rear end, they are not super strong, but at the power level you are talking about, it should survive as long as you use regular street tires, no drag radials and no slicks. The biggest issue is the 8" rear end is if you want a Trac lok and some lower gears, which you will need with a 350 hp small block. The parts are available, but I am not sure if I would want to put money in an 8" rear end, especially if I plan on ever really running the car hard at a track. For a pure street car that will never be seriously abused the 8" at that power level will be fine. If you have any inklings that you will later upgrade the 302 to a 351 or a big block, put a manual transmission in the car, or add some type of forced induction or anything that will put you over 400-450 hp, I would ditch the 8" now and not spend any money on it, and get a 9".