You are one of the few people who, like me, wanted to keep the existing water heater if possible. Most people seem to prefer to replace it.
On my '67, the water heater slid out the outside once all the exterior screws were removed, and three interior trim skins were removed.
To test it, fittings were made to hook it up to city water pressure, and the tank was allowed to sit overnight with pressure. You can read more about my water heater here
p.s. I just finished my second season of camping with the original water heater.