I have a 68 F100. Has a rebuilt 87 302, Holly carb, and a c6 transmission. When I got the truck, there was no down-shift (detent) cable. Original motor was a 6cyl. Would a universal cable work in my application? Or do I need to try and find the old style shift rod and modify it? I would really like to try this ride down the road with the cable working.

 

I would need to do some more thinking/research as to whether the detent is moved forward or rearward. If you have it attached to the throttle arm on the carb, pulling against spring tension a cable could work if you could anchor it properly.
It gets a little difficult to set up a cable to push something although it is not impossible.
You might wind up looking for or fabricating a rod set up.