Years ago back in the early 1990s I built two SB motors for a NHRA 340 SB stock car 1971 Challenger racing family in SO CA. One short block was a freshen, new pistons and rings, new cam shaft and lifters and freshen the X heads, that motor had been built by Greg Luneack originally and it was around half filled with what looked like cement. The other motor was all new parts starting with a new R1 block(legal for NHRA) and a different set of J heads that they had. We put in all new valves, springs keepers and retainers on both sets of heads.
We dyno both motors and found that the filled motor on the dyno had two different distinct heat zones on the block, above the fill the water temps. measured with a infra red gun, was 120 F, below the fill was 180 F I haven't filled or advise any one since to use block filler, never, no more :tsk
Cast iron expansion will follow the heat , hotter expands and cooler contracts more, not what you want on a cylinder in a running motor, especially a race motor
The non filled block ran faster at the race track with the same heads and camshaft as the filled motor did with both motors having the same brand pistons, bob weight, C.I. and compression ratio
Cement based fillers shrink away from the cylinder walls after curing, the other type plastic based filler maker says it doesn't do that, maybe it does and maybe it doesn't
EDITED: Some people swear on using block filler, I swear against using it
AKA aside, I won't use it again, no matter the circumstances