We tried the sledge hammer route, scoring first with an abrasive blade.
I wasn't surprised when we didn't get very far with too much work.
The sections vary from 5/16 to 9/16 thick, with the occasional rib. At
one spot I gave 4 mighty (!) blows with a 12 lb sledge & it didn't even
crack.

But I did get the OK for the plasma cutter. I brought a couple of
pieces home to try & it cuts pretty much the same as equal thickness of
steel, as you would expect. My cutter is pretty much entry-level, so it
will be pretty slow, but still faster & much easier than the sledge.
With a 20% duty cycle, I figure that I can cut 6-1/2' of 5/16 thick per
hour.

Now I gotta go find a portable compressor, or more, that will keep up
with the torch. Maybe the torch's 20% duty cycle will help.

Bob