removal of speedo cable from speedo in 74 Dart

How it was done back in the day... Disconnect neg battery cable. On your knees in open door jamb area. Remove screw from fuse box and move it a little. Remove stem and bezel nut from headlight switch and drop it out of the way. Arm goes in through this space created. Pull the cable/sheathe to under the dash. Now you can pull the cable out of the sheathe and lube it on the way back in but,,,
the sheathe does a 90 degree turn under the drivers seat area. Pushing the cable back through that turn can be quite difficult. Its a lot easier if you disconnect the cable at trans end and straighten it out beyond the one strap holding the curve. You can flush the sheathe with parts cleaner and compressed air now that its disconnected too. Reconnecting at speedo isn't so difficult even though you can't see well what you're doing.
Cleaning and lubing was common service, would help back when they were younger and had issues only in cold mornings. 40+ years old probably should be replaced. You'll be wise to pull the inst' panel so you can clean 40+ years of hard grease/crud from the instruments square input port before poking a new cable in there. Of course all this varies with how many miles it has been ran. Your 74 have 42K miles on it or 102K miles on it? Good luck with it.