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The source block can also sit on the lava. The only source/non-source issue you have to worry about is for the lava. Source lava blocks create obsidian, flow lava blocks create cobblestone. As long as it's a source lava block, it'll turn into obsidian if it touches any water, source or no.
The thing you want to be careful of is putting water IN the lava, for lack of a better way of saying it. When I make my molds, I make the dirt layer for the lava and then I put another layer on top of that. It's easier to put the water in if you can just aim for the inside of the layer above the lava. If you've only got the mold built up to the current lava layer, then you've got to worry about water spilling over the side if you don't re-bucket it fast enough, ruining nearby torches, tracks, etc.