Casale Engineering makes v-drive gear boxes.  One of their latest products is an add on prop release that mounts between the gear box case and the "in-out" housing.  This addition is a MUST for the serious v-driver.  Although Casale pioneered this devise it is now available from others.

     Most popular today is the split case v-drive, but Casale first came out with the "top loader" design.  To obtain a gear ration"quick change" capability, they made a square box with the back removable, but by that time the split case had really cought on and this design was dropped. 

    Casale has made several one off special designs in the 60s capable of taking the power of an Allison v-12 in both v-drive and z-drive configurations.

Casale Engineering