4 wheel disc brakes
This is mostly for you linelock question.I did 4 wheel disc brakes on a 65 Belvedere with line lock. Instructions in the above posts,, Install line from MC to line lock solidnoid. From line lock to left right spliter for front brakes. Rear MC to adjustable prop valve directly to rear brake line.Now this was on a 65 so It was replacing a single master cylinder set up,no factory prop valve that used a 3 way spliter,,2 lines to the front brakes and one to the rear that was plugged off,now using the adjustable prop valve. On a 69(dual master) I believe it has a block with a brake light switch,set prop valve from the factory. I wish I had a 69 service manual to see if you need to bypass or if you can use it for the rear brakes. I'm sure some one will have that answer.