Timing chain covers from 64-68 used the 8-bolt water pump. '69-79 used the 11-bolt water pump. If you have a pulley alignment problem you need the correct depth pulley like the above post said. You will also need a different alternator bracket (from water pump to the adjuster bolt on the alt) since the location it attaches to on the 11-bolt water pump is slightly more to the pass side than it was on the 8-bolt pump. These brackets are sold by PY, Ames, Year One, etc. Your crank, A/C, power steering and alternator pulleys can remain as is.
Here is a link to a page that will explain better:
http://www.hoigto.com/tech/pulley.html