I converted my brake system in the same way in which you described. The push rod is the same size for each, and I had no issues in that aspect. The 67-72 used either a proportioning valve or a distribution block (i cant remember which one) in the brake system. I did not include it in my replumb. The biggest problem that I had was getting my rear brakes bled. I gravity bled them for an extended period then had to bled them in the traditional manner multple times to get the remaining air out of the lines. The brakes work great, and I have no fears of losing all stopping ability when a wheel cylinder goes out like it did a few years back.
