305 on top and 350 below

1992 chevy caprice 5.0 swap to 1992 chevy Silverado 350
my question is what is the name of the insert piece that is attached to the center of the rear main seal. On my 305 its just and open port to slide the torque converter in but on my 350 motor it looks like it has some kind of gear or spindle inside of it. The center piece where the mark is, and also I had one guy to say I could take this piece out and should fit right into my torque convert or even leave it in.
I'm a newby at this but to me it seems like something where a 5 speed transmission would go

 

Looks like a pilot bearing. Some models use bushings.The torque converter has to fit into the notches, and I dont see any.

 

So my next question is, would I have to remove the pilot bearing to run the motor with my 700r4 transmission or could I leave it in and just bolt everything back together.

 

I am not certain on that. If it were me, I'd pull it out. I'd also make sure that you have the correct flywheel too. Make sure the teeth all look good, or toss it.