First off, you need the Mark V shell. If you have a wide body Mark IV shell, you will have to alter the front face plate in order for the chassis to fit properly. I had both Mark IV combo and Mark V head. Wanted to swap chassis. Mark IV chassis will fit into a Mark V head shell but not the other way around due to the shape of the chassis just below the control surfaces. As for the mushroom post to support the transformer, it is actually not for support of the transformer but for the chassis to prevent vibration. It is not essential or necessary.
I would also inquire if the head shell comes with a reverb tank already mounted. There is a difference between the reverb tanks as the one in the combo only has a plywood cover on the tank, the one for the head needs to have the aluminum sheet metal cover in order to install it.