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.
So I got a little nostalgic about the slant six and came across a good vid that has more than you ever thought you'd know about the engine. Turns out I was wrong about the block not having enough meat to bore it out. He says the original block was actually designed to be cast aluminum and so was plenty thick when they made it of cast steel.