I was in the same kind of situation myself about 5 years ago, and my '67 was my daily transportation at the time. I did some checking and found that there is only a very minor difference in the '68 centerlink. I have had a '68 centerlink on my car since - they are much cheaper for some stupid reason. It put my steering wheel off center and I just had to pull the wheel and re-center it. Keep in mind it's been 5 years since I did this, but I think the only difference was where the pitman arm attaches was moved just a little. The alignment shop had no problem when I took it in for aligment after replacing the centerlink. Keep in mind these are just my results and your results may vary.
Use at your own risk.