Pretty much this. I’d oversize the cut and grind down as others have said.
I don’t want to be the wet blanket here, but think about this project long and hard if the goal is truly a functional blade. You are talking about hundreds of dollars in material to start with, hours and hours of rough grinding, devising a way to heat treat it (we don’t even have a forge that can come close to getting something this large to the right temp., or quench, or etc…), or (probably better) sending it out to heat treat (which will be more money but has a vastly better chance of working), then you have final grinding, polishing, sharpening, final finishes… This world be a 15 part-er from Alex Steele and he’s a pro with a shop that puts our metal/blacksmithing capabilities to shame; he’d be the first to acknowledge that even with all that- it might not work.
All that said, if you decide to do it anyway, keep us posted! I’m super curious about how this’d turn out. As for materials, decide on an alloy and get some quotes on a piece that fits your dimensions. 768 square inches of 3/16 tool steel will be expensive and prices will likely vary fairly widely between suppliers.