I had a couple of slant sixes, one in a Valiant just like that. Well, looked like it anyway. They are torquey. That one is packing a Roots, I'm sure, among other things. Probably the only thing original is the block, and it's almost certainly sleeved.
No doubt, although it was already a long stroke and there isn't much block for boring. You don't get that kind a power without a Roots blower (I'm guessing it's side mounted on the engine so it doesn't stick up through the hood), either that or one helluva turbo/injection/intercooler setup.