You need a dielectric union to join the dissimilar pipes so no electrolysis can occur. That will prevent the corrosion you see in the old setup. The galvanized pipe can be attached to the new copper and the extra 2" can be on the copper side.
If you know what you are doing (and have the tools) you could also use PEX piping. However, copper is reasonable and durable.
Here is a briefing on how to install the dielectric coupling: