I'm going to assume you have a block that has been heavily decked. Aftermarket blocks have very thick decks like you're trying to achieve, but they can get away with it because of cast iron's ability to transfer heat. Your typical block filler works much better as an insulator for heat transfer rather than a conductor. If you were trying to save a hemi block or something, maaaybe you could get away with molten aluminum poured in since it would be a little better for heat transfer, but I'd only want to try it on block that would be junk otherwise.

Disclaimer: I'm no expert, just speculating.