What I did on the '3 was to add some smaller wire directly across the ammeter. Played with length & number of strands until the reading was about 1/2 the actual current. (Used the headlights as a dummy load while adjusting the shunt.)
There may be a some variation between ammeters, but a friend measured his TR6 ammeter for me some years ago, and came up with 2.7 milliohms. That means an 8" length of 16 AWG wire would make a suitable shunt, to make the ammeter read 1/2 the actual current.
However, I think we're jumping the gun here as far as 'upgrading' the ammeter. Even with just the stock ammeter, you should see the current start to fall off after just a few minutes of operation (depending on how badly discharged your battery was), and eventually go to zero with sustained driving. If it's constantly showing charge, you may have something else wrong, like a shorted cell or a bad voltage regulator (inside the alternator) or a wrong connection. The consequences can be severe, so I would want to first check out the charging, and worry about the ammeter later. Check the voltage across the battery terminals with an accurate meter. If it's over 14.5, there is something wrong with the alternator. Or if it stays down around 12.5 for very long with the ammeter showing heavy charge, likely the battery has a shorted cell.
Another test is to trickle charge the battery overnight (or otherwise have it charged), then install it and start the engine. In that case, you should see the ammeter current fall to nearly zero within seconds, and fully zero within a few minutes (of constant fast idle or above).