Jay the back isn't very pretty and there was nothing modified on the tach.
I pretty much just hacked a big hole in the area the clock would be, but just big enough for the housing to fit snug and be centered in the opening.
Then I slid the tach in with the ring clamp mount attached. The clamp was adjusted to seat against the inside of the cluster housing so when the cluster is assembled the tach is held in place.
The printed circuit needs to be left intact for the dash lights to work, so I insulated it with tape where it lays on the tach housing. I would have prefered to trim it off but that one run for the dash lights prevented it.