Sounds to me like you need to find a larger scrap yard. The yards near me use a track hoe with shears mounted on it. The shears are like giant hydraulic pinchers. For a large quantity they ignore the 3' rule.

If I had to cut it up on site I would try a gas powered cut off saw. Sort of a chain saw engine with a chop saw blade. I think the one on our truck is built by Husqvarna. I see construction crews commonly using the Stihl saw.

Good luck and let us know how you handle it