I've read/heard about people doing exactly what you planned on....putting the 'hot water heaters' in line (with bypass/isolation/check valves). IIRC the guy was a plumber so he regularly replaced peoples hot water heaters so he had a supply......My 2c worth.....if you are 'getting' old/ discarded tanks....be carefull....they may be corroded inside....there may be more problems than you can see. I'd offer what some of them may be but I'm not very knowledgeable about them. I did just 'flush' mine of built up sediment so I know things do build up. Be careful.