To be honest, I am not really sure if fumes would be an issue. I think it should be ok since the tank will be able to vent out the hose connections. That is if the hoses are disconnected. Beyond that, diesel has a relatively high flash point and it would take some hot sparks to iginite it in air (e.g., >100 F ?). If I remember correctly, diesels run on compression not on an ignition source. That is no spark plug, so I am thinking the sawzall should be ok. Have you tried calling a diesel garage or scrape yard?
I just pulled my tank this weekend with the intention of installing (cutting) an inspection port in about two weeks. Hopefully, I can let you know how cutting the port goes
. Seriously though I am not to concerned since I was able to drain the tank down to nothing using a jabsco hand pump and no fuel was in the tank while pulling out of the hold.
My question to you is how are you going to install a new tank if you cannot remove the whole old tank?