Hi Dan,
You'll definitely have to do some modifications to the plastic cluster housing to make a factory/reproduction tach and the associated smaller fuel gauge fit.
Here is a pic showing a gauge cluster housing and a gauge/tach cluster housing. They're older housings for a mechanical oil gauge but will still give you an idea of the differences in the tach and fuel gauge areas:
The dingy/dirty looking housing is from a regular gauge cluster and the clean white one (painted with Krylon Fusion plastic paint) is from a tach/gauge cluster.
Over on the tach side it looks as though you can simply cut the big opening and screw the tach right in. The mounting points are already there.
But over in the lower left corner is going to be a bit more involved. You'll have to cut away the baffle that used to be for the "BRAKE" light and probably graft in the chunk of plastic with the 3 little square holes for the gas gauge terminals that you cut from the other side. The small gas gauge shares it's lower/right mounting screw with the neighboring temp gauge. But the non-tach cluster doesn't have a place for the screw that holds the upper/left corner of the small gas gauge so you'll probably have to make something there.
Now, if your existing cluster has a clock (instead of a "BRAKE" light) down in that lower/left corner it might make it easier to fit the small gas gauge in there but I don't know for sure.