Are you sure you just don't need to lubricate the speedo cable. A year or so ago, the speedometer on my 68 buzzed and jumped around. A little lubrication and it's worked fine since. Your local parts store should have the lubricant (It's got graphite in it... although some folks swear by white grease and a low viscosity oil combination.) The point is you will have the cable out as it is, so if it looks dry, try the cheap fix first.