Stupid pre-buy questions
A youtube shows a guy flattening a 4 cyl head by just sanding. For the final work, he spray-glues ~320 grit to a thick glass plate and slides the head back & forth, using its weight for force. Takes ~30 min to change from ~6 mil warpage to flat within <1 mil. Cast-iron would take longer, but the manifolds have much less area to remove. You also don't need the flatness of a head. I recall photos of slant manifold gaskets that are quite thick to handle warpage. As mentioned, you would want to first tighten the 3 attaching bolts with the faces resting on a flat surface.
For final bolting to the head, loosen the 3 bolts, snug the nuts to the head, snug the 3 bolts, repeat several times so all parts find their best home. Definitely read the slantsix.org link Dan posted. You orient the triangle and cone washers opposite from what intuition might suggest. If not, your exhaust manifold can crack, which is a common issue in all straight six engines.