FWIW, here's what I did recently.
I replaced (not cleaned) my elbows - they are expensive parts
, but considered a wear item. Yours may be cleanable, but if they are more than a few years old, you'll have to make a call on that.
For the manifolds, I took them along with the heat exchangers to a radiator shop. They cleaned them using a fairly mild acid (I don't remember what, but they said it was much less than muriatic), and rodded out the exchangers. Came out very clean. I also took the manifolds to a machine shop and had them resurface where elbows mount, as mine had some rust build up.
I replaced all the gaskets as well (elbow, filler, manifold, and bottom bracket), and then primed+painted everything using high-temp engine paint
(I chose "New Ford Gray" as a relatively cheap