|I just did mine on my 1987 Mk V. There is plywood under the fiberglass. I just drilled new hole, plugged the old ones, and used the old lag bolts the old head was bolted down with. Reconnected the hoses, and away it went. Took less than 1 hour. There is a thread somewhere in the Black Arts showing a picture of a Mk V where that owner drilled approx 2 inch access holes in the vertical face of the platform the head is mounted on to get at the underside of the surface - he found some plastic plugs used in electric situations to act as removable fillers. I did not find that necessary.
Barry, Oasis, 1987 MkV