Re: Not mentioned but...
You didn't say what year car this was in. If your Z is a first Gen (70-78), and you are importing a 5 speed from another year, it is vital that you buy the correct shiter lever.
A four speed shifter and a 5 speed shifter are very different.
The distance from the pivot to the shift engagment is much shorter on a five speed. A four speed lever does not engage properly. Many have been able to get them to work, but it just doesn't work as well as the five speed shifter.
Also, if you imported the shift lever from the 5 speed tranny, and it came from a 2nd gen ZX, the lever is not shaped correctly, and can interfere with the console. This can prevent proper engagement of 5th, letting it pop out easily. Some have heated and bent the ZX shifter to get it to work, but the lever is much shorter.
The best solution all around is to pony up the cash and get a proper 1978 5 speed shift lever. May not be your problem, but is a likely suspect.