Are you concerned with appearance through the wheels when parked, or may be more interested in real time braking? The reason why I ask is that we see this EVERY DAY, where people have installed an aftermarket brake kit that looks better than stock stuff through the wheels when parked BUT in many cases actually stops worse than the OE disc / drum brakes that were replaced.
So what am I saying? Real time street braking power that will flat astound you can be had by simply only installing one of our hydraulic brake assist systems. Day after day after day, we speak with people that have installed a name your brand 4 wheel disc brake system onto their muscle car reporting back either only on par performance or worse overall braking than the stock disc / drum systems they went through soooooo much time / effort / energy / costs to replace. Yeah, they look cool through the wheels when parked, but leave a lot to be desired when actually rolling down the road.
Freshen up the stock disc / drum system as needed to bring it up to snuff, then install one of our hydraulic brake assist systems and call it done. Unless you are really going to put a helmet on your head and go run a high speed road course, the stock disc / drum brakes will do exceptionally well. You could even put a set of the Wilwood D52 style replacement calipers in place of the stock ones if you wanted to still see something more than stock in the front. I have taken countless amount of people for test drives in our shop 442 (that has factory disc / drum brakes in it driven by one of our hydraulic brake assist units) with them reporting it is the best braking they have ever experienced (despite many of them already having big high dollar brakes installed on what they drove in on).
Price out a complete freshen up of your existing factory disc / drum package, including everything (new rotors - maybe even fancy drilled / slotted, new brake flex hoses all around, fresh calipers or even the aftermarket Wilwood D52 replacements, upscale ceramic pads, new drums, wheel cylinders, deluxe complete rear brake hardware kit...), add one of our hydraulic brake assist systems in with a new clear coated oven baked master cylinder against what you may be looking to spend on your 4 wheel disc conversion?
Do also note that a rear disc brake conversion is going to require that you address the very real likelihood of excessive rear axle shaft end play - if you find more than .020" (which I bet you certainly will), factor in the costs to dive into the rear end to clearance out the end play for proper use with a rear disc brake conversion.
http://www.hydratechbraking.com/GM_A_Body.html