The threaded hole in the pushrod is there to aid in removing the pushrod, the pushrod goes through an oil seal about 2-3 inches from the opening that is covered with a rubber 'boot'.
This is what you're pulling against, this and the fact that the pushrod goes all the way through the transmission, inside the input shaft, so there might be some 'grunge' in there as well complicating the removal.
Normally you can grab the pushrod with a pair of pliers, and pull it right out, but it sounds like it's been a long time , if it ever has been removed.
Welcome to the world of old motor vehicles, where a simple 5 minute job turns into a weekend project !!