Great info above. Can I interchange the "quartz" borg warner clock into the 1978 tach for better time keeping? Are the pins the same? Can I put my blue face and speedo into the 1979 stamping to accomodate the clock, or will the tach assy. fit, and just change the face? Anyone do this?
The tach (and clock) mounts to a flat metal "plate" which then screws to the cluster casing. At least on the Late 78 - 81 cars. I don't have my clusters handy, but I believe the Early 78 and prior tachs mount in a similar same way. IF the bulbs, screw holes, printed circuit, etc all mount in the same locations, and the early tachs mount to a separate plate that screws to the cluster you should be able to swap the tach/clock assy. I've seen later tachs in quite a few early cars. You can denote the later tach/clock as the rear of the gauge will have an offset white plastic "oval" shaped casing over the "guts" rather than the round cylinder-type protrusion from the earlier tachs.
At the very least, you can get the entire cluster from a 78/79 and transplant your earlier gauges and circuit board in. This probably would be the easiest way to get the newer tach/clock in an older car w/out it.
The Quartz clock mounts in a slightly different way, and as such, the clock adjuster is in a different location. So you'll need the entire tach and a new plastic window to do the swap correctly. Otherwise the stuff won't line up properly.
If anyone can correct the above info, please feel free. This is to the best of my knowledge, as I don't have my spare clusters handy at present.
*EDIT: Actually, you should be able to find a later 78 cluster no problem and swap it right in, given you have a 78 to start with. The speedo just screws into the cluster by 2 small screws. But, double-check the wiring first and make sure the circuit is the same.