The gear shift will not go into first gear and the transmission will not shift into overdrive. The gear selector cable is attached to the transmission with a clip only and not the bolt shown in the Haynes manual that is supposed the keep the cable from sliding. The cable (which is too long and has been looped to tuck away ) is connected to the cable bracket by two nuts instead of a clip. I suspect the linkage is not adjusted properly and need to change the cable to the correct one. Am I on the right track?

The trans mount has little cracks in it and the trans is leaning to the right. I suspect both engine and transmission mounts are bad. There is the clunk when shifting to reverse as well. What is the part number for the ES mount to use? ES told me they did not have a "listing" for the 87 Z28.

 

ES had a few issues with some, but not many, third gen installs having the rear of the trans too high when using their mount, therefore, they decided to advise against us using it. It's the same part as for a late 70s F-car with the TH350.
These 3rd gens have had 2 different styles of crossmembers for the trans. One is just a single solid piece of steel, which is what you want, and the other has a second piece above the main one, with bushings between the 2. This design causes problems, and belongs in the trash anyway.

 

Thanks for the info on the mount, another problem solved. The cross brace appears to be a one piece unit and it is installed to where I can't acess the rear trans oil pan bolts. I'll have to play with that too.