Since its just one click with each turn of the key, I'd place my bet on the start solenoid. The little copper L-terminals inside are likely either worn down too far or unevenly and its not getting a good connection. Here's a thread of mine from the way-back-when machine:

Thread.

Best of luck to you! <img src="/forums/images/graemlins/kewl.gif" alt="" />

Sean Strawmyer
Back and ready to rock...... crawl.

From Indiana or surrounding states and interested in wheelin'? Check out www.mwior.com