GMCPauls only carries one size for our year of trucks, it's 70 inches long. LMC carries several lengths. The trick is how you are going to route it to determine length. A coat hanger (or two) comes to mind to help layout the path and estimate the needed length.
No tight bends and stay as far away from the exhaust as possible. I'm betting the one from GMCPauls will will be fine, but check your truck first to make sure.
https://gmcpauls.com/instrument_panel.htm
https://www.lmctruck.com/1967-72-che...dometer-cables
I can vouch for GMC Pauls parts and service...I have not dealt with LMC at all.