1990 Mustang 5.0 Manual Gear Shifter Is Stuck!

Umm... no. I wasn't being sarcastic. I'm just supporting your statement. But my post was 4 hours before yours. You just happened to agree with mine without having seen it, apparently. Don't be sorry. Saying the same thing as someone else builds confidence in the OP that the information they're getting is correct.

These time stamps are still screwy. It did not take me 4 hours to go check if I could shift my Mustang with the clutch pedal up and the motor off. If the OP matched RPMs, you might be able to shift without a clutch going down the street. Do not try that at home unless you have worn syncro's.
I think the "stupid" on the silly sign struck me wrong when I was trying to tell the OP the cable will not do it. I will try go grow some thicker skin today. A thicker skull is not helping.