I am going to be installing a disc setup from a '69 Elky onto my '68 Chevelle. At present, it has 4 wheel manual drum brakes. I will use the master cylinder from a '70 W-30 Olds 442 for manual disc brakes. Do I need a prop valve or anything else to make this work? Or can I just hook up the master cylinder to the existing lines leading to the original distribution block and go? Thanks!

You'll need the prop. valve from the Olds to get the F/R bias correct. Discs take a lot more fluid and pressure than drums, and a drum/drum prop. valve isn't metered for this.

Like said you will definatly need the prop valve, i just removed my dist. block all together on my conversion. All of my lines bolted (screwed) right into my prop valve, my prop valve is located on the frame right below the top of the floor boards (if that doesn't make sense can't explain it to well)

I don't remember replacing anything else, but i went with power brakes & added the booster.

keep the drum brakedistribution block (it isn't a "proportioning valve", as it does no pressure reduction on a 4 wheel drum setup) and use it as a junction block and allow you to keep your brake light hooked up. get a $40 Wilwood prop valve from Summit or Jegs and plumb it into the rear brakes- either before or after the stock junction block will do. i put mine before, simply because i needed to fab up a line from the master to the block anyways. adjust it so the fronts and rears lock up at about the same time, and then just forget aout it and enjoy the car.