Definitely check out the FAQ write-up on this. I just pulled and rebuilt the heater box on my 1974 and couldn't have done it without the instructions from the FAQ. My (newer style) heater valve started to leak, so I decided to bite the bullet and pull the entire box for rebuild. Glad I did because the seal around the top of the heater box was toast and water was leaking into the car next to the firewall and seeping down under the carpets. While I was at it, I had the heater core pressure checked and rodded out. One of the most worthwhile rehab projects I've done on my car in a long time. Good luck,
--John
PS, it wasn't all that hard to do.