Created a spawner room for skeletons which is a oversized 9x9 conveyor belt floor with a small drop through witchwater signs to convert to wither skeles. it's been running for maybe 20 minutes with a cleaver with beheading 4 and flux in a autonomous activator doing all the killing and I already have 7 stacks of wither skeleton heads.
Thanks for the tip. Just replaced my diamond spike with a full shiny cleaver in an AA - already up to 40 skulls.