Will a throttle cable stretch over time?

I'm sure these cables stretch SOME but I don't see how they would ever stretch enough to get into an "80 some" percent deal. Look at your clamp setup near the carb. There's a LOT of adjustment there. It's far more likely that something has been replaced or mis-installed over the years Photos most certainly would help.

As old as all these old girls are, even the ones in the 70's and 80's, anything is possible

Start by downloading yourself a free factory service manual here:

http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617

or here

http://www.mymopar.com/index.php?pid=31

Throttle /kickdown setup is in the Torqueflite transmission section. This does not show it very well, but this clamp allows a lot of adjustment on the throttle cable. Also check that the carpet is not interfering under the pedal