68 Javelin Disc Brake conversion

Yes, if you use ALL the parts from the donor vehicle as they came off. Bolt the donor axle stub, caliper mount plate, dust shield, rotor/caliper to your 68 Jav steering knuckle using the donor car bolts (or new Grade 8 hardware of the same length). Basically everything outboard of the steering knuckle. Keep your Jav steering arm which bolts to the inside of the steering knuckle. There are way too many combinations of axle stub length, caliper mount thickness, caliper offset and rotor dimensions to try a hybrid setup. Remember to keep a record of what brakes you're using for future service work. New wheel bearings and new flexible brake hoses are also a good idea during this swap, especially the brake hoses. Use new hoses to match whatever donor car the brakes come from, the Hornet flexible hoses should work fine since the hardline mounting point on the body is in the same place as on the Javelin.  Not sure about the Matador flex lines. I'm using a Hornet single piston disc setup on one of my 68 AMX's, works fine.
  You'll also need to swap the master cylinder and brake booster for one that will work with the disc setup if your Javelin doesn't currently have power discs. Cardone remanufactures a brake booster/master cylinder combo or you can find them individually. Also Google "Booster Dewey" for rebuilding service of power brake boosters. Hope this helps.