You said this was a six cylinder car didn't you? Putting a V8 into this car really requires a lot more than just dropping it in. The suspension on six cylinder cars is not as tough as the V8 cars, and you would either be better off just starting with a V8 car, or finding one that you can pull all the parts off of. The stock six cylinder suspension will be severely overloaded with a big V8 dropped in.
You probably won't have much luck finding original front disc brakes (if you do, they can be expensive to rebuild), but you do have several options. There are several suppliers (one of which is Stainless Steel Brakes--I think you can go to
www.ssbrakes.com --I think thats it, and check them out). I'm not sure how big a brake you can fit under a 15" wheel, but the stock ones were about 11" and can work pretty good.
The cheaper way to go is to swap the front spindles and brakes from a mid 70's Granada/Monarch. These brakes are also about 11" and can be bought very cheap. I think the only real change you have to make is to use the Granada outer tie rod in place of the Mustang one. You can probably put these brakes on the front of your car for under $250, that includes new bearings, rotors, and pads.
This is much cheaper than the aftermarket, which runs about $750 for the front and another $750 for rear disc brakes, which is why I suggest just sticking with stock rear drums. Either way, if you add a lot of power to the car, the tiny six cylinder brakes will never do the job. The V8 will add power and weight to the car, and those little drums were never intended to handle that kind of stress.
Remember, if you put safety first, you can enjoy hopping up your car for many years of street rodding, but if you put the power first, you may never get the chance to worry about safety until it's too late. Just dropping the engine in and hammering the throttle can be the recipe for disaster. Just do it right the first time, and you can forget about it. If you put a basically stock 5.0 into this car with a nice Holley carb on it and a good exhaust, you should have around 275 HP to play around with, and if you have the rear gears in behind it (3.55 or 3.80) the car will be a LOT OF FUN, but a real hand full on wet streets (trust me, I know about this part).
Have fun and good luck.