'68 w/ stock Rally Wheels. Brake Conversion

[Berg]

I know this is a few months old but I did a little bit of research on this. There are companies out there that offer full front power disc brake conversion kits in different styles. There is an OEM-Factory correct reproduction style that uses an 11" Delco-Moraine stamped booster, Correct style master cylinder with bleeders in either square or rounded style, new hold-off valves and the correct proportioning valve. You can order a new OEM hard line set from places like inline tube or Master Power or any other kind of supplier like ground up or east coast just to name a few. I believe the 68 Chevelle disc brakes originally had twin piston calipers but most replacement kits come with the single piston which you are probably better off with. If you have manual drum brakes you could get the whole kit to convert it to power or you could get a manual brake conversion kit also. I'd go power personally, you could also get a smaller dual-diaphragm booster if you are worried about clearance if running a big block.
There are also kits that come with conversion lines and use a 71-72 style combination valve instead of having the factory style separate small hold-off valve mounted to the booster/master. If you are going for originality that would be the way to go in my opinion. If you are just looking for a safety upgrade and don't really care about factory correctness there are generic vacuum boosters and master cylinders. You could always run a GM truck aluminum master cylinder for $20 as a reman at a parts store.)
There are kits on the market that have a disclaimer that they will only work with 15" wheels or larger diameter. I looked at a car with one of these kits installed and the guy did a massive amount of caliper and caliper bracket grinding to make his original wheels fit. Some kits offer cross-drilled and slotted rotors as an upgrade to their kits. Be cautious of which brake lines you use if you get a conversion kit. Some kits need their own special lines and front brake hoses due to the fact that they reuse the original frame brackets and due to the location of the combination valve. This could possibly effect you later on if you ever need to replace a line.

Another option is to buy a partial disc conversion kit that comes with everything mounted between the A-arms and then just add your own hold-off valve and put an in-line adjustable proportioning valve for the rear brakes.

Finally, there are also hydraboost conversion kits that use the power steering pump as a power assist. GM did this a lot in trucks from the 70's to current. This is a good option if you have power steering and you run a big cam that keeps you from building the proper amount of vacuum for a normal booster to function properly.
This is just a quick summary of my research and all off the top of my head so i'm sure there is some stuff missing or not explained well enough. Good luck with your conversion. It should be possible to have it all done for around $400 total all the way up to around a grand for everything brand new in your brake system. complete with stainless lines and stainless braided flex hoses with all new clips/distribution blocks, and rear wheel cylinders also.