I will say this, I have not found any aftermarket speedometer cables that have lasted any length of time before they started to make my needle bounce at low speeds. No matter the lube used they seem to wear out really fast. This last go around I purchased an AC-Delco cable (still made in US) that I think is meant for a late 90s full size. Classic Industries lists it as correct for 70s Firebirds with manual trans. I think it might be a little longer than factory as it is over 80 inches, but it fits nicely on my 72. But mine is an M20 with passenger side exit and I route over the bellhousing. More importantly to me, it came pre-lubed and the speedo is dead steady even at a slow crawl and believe me I have tested it in Southern Californian traffic at rush hour. The classic industries part # is G8793. If you want I think I have the ACDelco part number in my garage book, then you could get one from other places.
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I built it one piece at a time and it cost me more than a dime. 
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