I want to put front disc brakes on the front on my 67 Impala. I have 15" Rally wheels on it which I know is required. Would I notice improved stopping staying with manual brakes? Or would I need to have power brakes to notice any difference? My question is there any modification that I would need to do to mount the booster to the firewall and the brake pedal?
From what speed are you trying to stop? If it is above 60 mph then a disc brake may cut a few feet off of your stopping distance. The factory built your drum brakes with the ability to lock up the wheels at any speed. Once locked up you are skidding, but without computer controlled ASB system a skid is the best you can do.
With Disc brakes you can stop from a higher speed of about 85 mph without any fading with the stock 11 inch rotors. Or if you are into road racing your drums can be upgraded to discs as a disc brake can shed the heat of the brakes faster than a drum brake can.
What you are looking for in a brake system is a lot of metal to absorb all of the brake heat and slowly let it radiate away as the car drives further down the road. Disc brakes are massive weighing much more than a simple drum brake. Of course in a road racer you don't want a massive disc brake in the wheel as you want that as sprung weight, which is why on Jaguars and many other high priced sports cars the disc brakes are moved inboard so that the shock can more easily control the mass of the wheel and tire.
Most disc brakes sold as kits are Metric disc brakes taken off of a late eighties G-body cars (Monte Carlos). The earlier 1969-1978 A-body discs are about gone now so they are using the newer though smaller in size disc brakes off of a lighter car to install on your older car as a kit. If these parts arte new then odds are that they are from off shore as well using parts made in India or Pakistan (that sell a lot of low cost brake parts).
Bigger than 11 disc will require a bigger wheel size. The 12 inch disc rotor used on Camaros required a 16 wheel. The 14to 16 inch discs used on Corvettes require a 18 to 20 inch wheel to clear. Wilwood and Baer also have larger than stock wheel requirements when install
As far as Power Assist goes it also will not make you stop any faster. It simply allows you to hold the brake pedal while stopped at a light with less effort as disc brakes require about double the line pressure of drum brakes to work.
For less pedal pressure go to a power assist. You have to change the activation rod, and the hole on the brake pedal that it bolts into as well as the master cylinder when you add a power assist vacuum diaphragm. (plus the hose, fittings and one way valve to maintain vacuum in the can). That will give you an easy to use brake pedal but it won't stop any faster.
Larger heavy duty brakes off of a cop car or off of the same year station wagon will give you better stopping power but you have to replace everything as the y use a wider drum to fit wider shoes and in the case of the station wagon rear brakes even bigger drums. This means changing the backing plates which are not reproduced (which means scrounging junk yards across the country once you find the Hollander number for those brakes that the parts catalog excludes).