[olddog]

The engine is sitting on two mounts. I would assume you had to set something under the back of the engine to support it, when you removed the transmission and bell housing. If you start the engine, you have to make sure that whatever is holding the back end of the engine, doesn't flip out from the movement. Without the transmission and mounts attached, I expect things will be free to move more than normal.

Go to the trouble of centering with a dial indicator. If it is spot on, your done. If not, good thing you checked.