I'd like to do the 3g upgrade, but I dont want to pop the ammeter, and then have to find a place for a voltmeter, and worry about burning the harness.
The rest of the harness should be alright? right?
Do the upgrade if you want to anyway. The ammeter and original charge wire are not an issue because, if you do it right, you won't be using them anymore.
Yes, the rest of the harness will be fine, assuming it's fine now that is.
With the 3G upgrade, you completely bypass the original charging "loop" wire and run a single, larger gauge, wire from the alternator's output post directly to the battery's positive cable.
Then you cap off, or eliminate completely, the old charge wire that used to connect to the alternator. That way it won't short out on anything (since it's still powered from it's other end).
This is why the ammeter no longer functions properly. You're eliminating it's signal source.
I'm not sure if by "pop the ammeter" you meant electrically (by putting too much current through it), or physically (popping it out), but it's not the ammeter that can be damaged by running too much current, but the wire itself that can melt because it's too small for it's length.
Paul