Why do you want to use fill in your block?
Have you had it sonic tested? If not I would
If it has at least .160 cylinder wall thickness after boring on the major and minor thrust sides of the cylinders DON'T use hard block or any other filler
All fillers end up creating two different heat zones in the cylinders, NOT good
I've made over 800 HP on a stock 440 block and a little more on a 400 blocks with no filler with no problems sealing or living at the track and on the street