Best thing you can do is take everything. If you have room, park the donor car in the garage too until you're all done.
I'm going to be swapping an LS1/4L60E into a 68 Camaro. There are many things to consider outside of the engine/tranny itself though. You have to think about custom mounts, headers that will fit the new engine, yet clear steering and suspension on the old car. Fuel system is another consideration. Most new fuel injection systems don't deadhead . . . or rather, they have a return line back to the tank which many older cars don't.
I bought an engine/tranny combo from a 99 WS6 firebird for 2000 shipped to my door, but I still have another couple grand to go to just getting it into the Camaro and running.
Bringing history and technology together.