I've done it both ways and I can say this: for my brother's z we left the trany attached since it was quicker and less work. For my Z, separately because their's less change for scratching up the engine bay and because you need 2 pairs of eyes. One person working the hoist from the top and the other guiding it in from the bottom.
There is no glory in preparation & practice, but without preparation & practice there is no glory. So do it for the "Z"... and GLOOOOOOOORY!