just curious;
what happens if i install an extension box and a stern jack together on the same drive?

If it worked your other problem would be stress on the transom. I had the IMCO extension on my boat whick looked almost like the land & sea and it did cause some stress cracks because of the way my transom was finished.

-stress on the transom

good point ,actually i thought this one but i said it may handle..

You could always stack 2 or 3 SternJacks on top of each other...