I wouldn't waste my time or money filling any water block for drag racing or any other use
One of the main reasons I'm against using concrete based fillers is all concrete shrinks while curing, pulling away from the block in all ways, cylinders and all other surfaces.
Another thing about partial fills is the block heat being differently above the fill than below it., I dyno a 1971 340 NHRA stocker motor that Greg Luneack (SP?) built for the customer, that block was about a 3/4 fill. it was slightly below the water inlet holes on the front of the block from the water pump. They like to race it with 130 F water temps so we were doing that on the engine dyno, we used a laser heat gun to measure the tempts of the block and heads, the heads read 130 F after several pulls and the block read 180F below the water in the block down in were the fill was
Not good on the ring seal, HUH
There are some composite fillers that may not shrink when curing, but which type and brand I have no idea of and I have never heard much about them either
How much power are you wanting to make?