Leave the distributor out until the engine is in. Less chance of hitting it and breaking it, easier to get to the top 2 bellhousing bolts, easier to hook up the oil line if you're running an aftermarket mechanical oil pressure guage or easier to install the sender if it's electric. If you're installing engine & trans together and this is your first time I can almost guarantee you'll smack the distributor into the firewall & mess it up. Don't know how much other stuff you're replacing but it's a good time to install a new speedo cable and if so push the rubber grommet into the hole in the firewall while the engine is out of the way. Saves on bloody knuckles later.