I am reading this as saying: You are happy with losing the ammeter as long as you get the tach feature. So, you intend to discard the printed circuit that came with the cluster pictured above, buy a new one applicable to your 76 (with tach option) and install it in the early model cluster. When that is all done, you are going to pull the ammeter from the cluster and replace it with your current voltmeter.
Sounds like a plan. And, it doesn't look like the circuit board change is even necessary. See the attached dwgs. Note that the pre-76 boards are layed out (even with the tach option) exactly like the 76+ ones - and the harness pinouts are nearly identical. There are a few differences, but they can be ignored for the most part. Leaving the 75- board in place and plugging in your 76 harness:
1. Gas tank level - your 76 non-tach harness should include a jumper that connects the two 30 pins. So that won't be an issue. Just snip the lead that goes to the unused 30 pin - on the 73 with-tach board.
2. Grounding - the 73- boards were grounded by only one lead (150) while the 76+ boards used two. Disregard the dedicated voltmeter ground on the 76+ boards for now - it will be used to power the ammeter.
3. Seat belt warning lamp - didn't exist in the olden days. Snip the WHT 237 lead on your harness.
4. Brake warning light - as with the gas tank sender leads, your 76 harness should have a jumper between the two 33 pins. Snip the unused one.
5. Ammeter to voltmeter - no changes required. The pins that supply the ammeter clips (105 & 106) in the 73 are in the same locations as the ones used to drive a 76+ voltmeter (39 & 150). Just switch the meters and that should be it.
Wiring diagrams for 76, 75 & 73 clusters are below. I attached the 73 because it is clearer than the 75 - and the wiring is identical. BTW, I wrote this pretty quickly - so be sure to check my work:
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