414 block half fill in waterjackets

IMO - no. It's not a street engine anymore. 1/2 fill normally means to the top of the freeze plugs because it's 1/2 the height of the cooling jackets - that's roughly the lower 3rd of the cylinder core. If you want to try it - I'd run an oil cooler because the heat transferred to the cooling jackets in that area of the block is from the oil.