Any guesses why it left me stranded?

Asus_

Oh, your golden. Engine swap that beautiful Z & do it right the first time. Take your time with it, find a mechanic you feel comfortable with.

I think maybe those horror stories might have been from engine swaps for different vehicles, which will be more complicated. You are going Z engine to Z car so its the closest "Plug n Play" so to speak as you'll get . As victorofhavoc said, any competent mechanic, whos had experience with engine swaps will have very little to no issues with this.

As a bonus if there was anything mod wise you wanted, now would be the time as the engine will be out and will save you a ton on labor.