Plumbers cut cast iron pipe with a few different tools, cut off saws,A chain cutter you wrap around the pipe and snap it, sometimes a Porta Band. I've used plently of gas powered cutoff saws, they slice through 1 inch hardened rebar in seconds,takes a minute or so to cut a 4 inch schedule 80 steel pipe.
I believe Fenders is talking about a cast iron sheet not pipe? Use a cut off saw,it'll take a bit of time.Cutting 4 foot through 5/8 material will take a long while with a Sawzall and yes a few blades.
If it's 5/8 pipe,an unusual size, I could buzz through it with a hacksaw in 5 minutes.
