Speedometer cable

Rock auto lists 2 cables both the same and both listed for slant 6 and 340 All transmissions manual or automatic.

Either the cable you have is wrong or the spedo gear adaptor is wrong. Rock auto says the end should be 5/8" looks like even back to 67 same cable is spected for some applications. Up to 76 same basic info

Did more digging and it looks like there are two sizes on the trans end 5/8 and 7/8.

The link above was very helpfull