Listen to your machinist I use to help Gene Buhl, NHRA E/SA record holder and C/SA record holder , once he went to non filled early(pre 1971) blocks the motors set records I built several NHRA stock legal 340 motors for Steve and Linda Myers in the 1990s, one motor they had a new motor with a R1 block, their old motor was built originaly with the block 3/4 filled, the new block wasn't. Both blocks where bored to 4.080 and the unfilled block made 10 HP more on the DTS engine dyno at the same RPMs than the filled block did with the same cam degreed the same, intake and carb. on both
I did discover that the filled block would have two different temp. zones in it, above the cement would read 120 to 140F when warmed up to what they normally had before a run and the block, after several pulls on the dyno, would read 180+ F at the filled line and below with a infra red temp. gun