Can I break in motor with no driveshaft??

The problem with not having a yoke is that there's nothing to center the mainshaft. Either a stick or auto will try and turn "some" in neutral. I say figure a way to hold the yoke in place, like, a length of threaded rod with nuts/ washers.

I also say that you cannot PROPERLY break in the rings without some load on the engine. The cam is a different story.