Wiring upgrades and ammeter operation

I did essentially what you're talking about I believe. I basically ran parallel charge wires from the alternator to the batter. One was a direct line from the alt to the battery, the other was the stock wiring that went from the alt to the ammeter and through to the battery. I drilled a hole in my firewall and ran the ammeter wires directly since my bulkhead was a little melted though. What others have said is correct. You will still get a reading from your ammeter, but it won't necessarily be correct. It will only read the current that is passing through it, which depending on wires sizes and accesory loadings, will vary.

The tough part with fully bypassing the alternator is that one side of it is the main power point for several interior systems. If you look at a wiring diagram (at least for my 67), there are three or four systems that draw their power from the black side of the ammeter. Disconnecting it disconnects all those other systems, so you have to supply them power somehow.