1 lb or higher would obviously work better, but it can be done with 1/2 if you practice first on some scrap cast iron pieces.. not hard to find some spare pieces to practice on, pull some manifolds off a dead car at any junkyard.
Only difficult part of this process is finding a TIG welder/220 outlet..
Sounds like you're talking about welding up a very thick casting with high carbon content...which in this case yes you're exactly correct and it would absolutely need to be preheated, perhaps even drill the ends of the crack first.. add more heat get it near red hot, use a nickel rod, and cool it very slowly..
If you try to TIG a very gray, or high carbon content, block you're likely just gunna create a weld that's going to shrink and crack..
You're likely aware of the thickness, but I can't imagine the block is thicker than 3/4" so either method should be plausible if done correctly.
Though many wold just torch it, stick it, cool it and pray.