A cicular saw for cutting metal will take a while and could be dangerous if it kicks back. I have a special blade I use for diamond plate aluminum, but would not think about using it on steel.
A sawsall would be better if you use it right Maybe even a small band saw would be a good investment. Oxy. Acy. would work too, but there will be some grinding involved after.
Plasma is the best way, but costly.