You can pull the cable from the TOP, at the cruise transducer...no need to crawl underneath for that.
The (other)cable from the transducer to the speedo head has to come out from the speedo side...

Thanks Joel

Had simular problem on my 78. it was a combo of the transmission gear was missing or broken and also the speedometer gears needed fixed.  Corevette specialties I believe in Maryland  did the speedo work had it back in 2 days wroks fine and local garage put in the  plastic gear in transmission. Did get a cable and ran it from the tran to teh speedometer and bypassed the cruise cable fist to see where the problem was and still hooked up this way, works fine, but  sometime will get teh cruise control back  up and running. To remove the speedometer in the 1978 its not too bad. a couple of screws for the clear plastic and then teh surround and I believe one for the speedometer and you can pull it out enough to remove the  cable behind it and  its out. It clips back in and just take your time putting back the surround and  your done. Good luck  Corvette light blue 78.