If it's just to mount a block on a castiron base, I'd drill four holes; two for dowels to keep the block from shifting, two tapped for small screws to keep the block from lifting off. Or, some kinds of rivet might be good (the expands-at-bottom type like for blind holes in concrete). Then I'd butter the holes and mating surface with RTV silicone and mist it with water (to start it curing) and screw the block down.
Preheat for brazing in cast iron is a nuisance. Takes hours.