Well as no body else has had a go.

After initial checking of the casting to make sure you have enough metal where you need it shim it up true on the surface plate and mark a datum surface such as the top of block and one end. Machine the first surface using the same shims as you did when marking out, then with that newly machined surface on an angle plate do the end so they are at 90deg to each other.

You can then use these two surfaces to mount the work while the bottom and other end are machined.

You can now sit the bottom of the block squarely on the surface plate and have a nice flat rear to mark out all your horizontal lines with the height gauge. Next clamp the block to an angle plate same bottom surface to the plate and you can use the height gauge to mark all the points going the other way.

Optical centre punch where the lines cross and then when you go back to machine the part you have accurate punch marks to locate a centre finder in.

This is another way with small parts clamp them to a 1-2-3 block so you can mark lines going one way, then stand the block on end to do the others

If you have a problem with getting a knobly casting to sit still you could glue it to a flat plate with hot melt glue (suggested on here I think recently for machining small parts), or hold it in a big dollop of plasticine. We used to have industrial sized 2lb packs of placticene as a stores item in the macaroni factory I assume for this kind of thing. Doesn't help for machining though.