Replacing an atmospheric vented tank with power vented
A client? As in you're charging money for this? Refer your client to a licensed plumber/gas fitter because this is beyond your ability. Depending on where you are, you could get into a lot of trouble working on gas without being qualified.
About a year ago I had to relight a furnace after changing a gas meter. When I went to relight it, something didn't look right to me. I stepped back and realized it was a high efficiency furnace vented straight into c-vent and then dumped into a chimney with no flex liner. The c-vent looked like Swiss cheese and the chimney was completely rotted out from the condensation. Red tagged the furnace, and a few weeks later I was driving by and the chimney (in the middle of the house) was being removed. Cost the people who had recently bought the house big bucks to make safe, but at least it didn't cost them their lives.
Don't mess with gas if you aren't qualified, especially if its someone else's gas!