It's probably not your speedo cable.....it maybe your driven gear??
If you have a TH350 tranny...try this thread....
http://67-72chevytrucks.com/vboard/s...d.php?t=345091....
If you have a Th400...the speedo housing is a little different, but the change out is about the same....let me know, I have some pics of when I changed my TH400 speedo housing and driven gear out recently!!!
Hope this helps