Continuing disassembly, I got the cats off. There's one turbo to manifold nut that's stuck, the one on the driver turbo nearest to the transmission. There wasn't enough of the triple square exposed for the bit to engage and I've managed to round the exterior hex a bit.

I'm thinking that drilling out the stud is going to be the way to go, then I can get the turbo free from the flange. Then hopefully there's enough meat left that I can extract the stud with a vice grips or weld a nut back on to torque it out.