Been a couple with similar plots:
“The Brain That Wouldn’t Die” (1962) A doctor named Bill Cortner keeps his fiancée’s severed head alive after she is decapitated in a car accident.
and
Steve Martin’s “The Man With Two Brains” (1983)...............
The Brain That Wouldn’t Die, that’s the one I was thinking of.
You forgot a good candidate for Mystery Science Theater:
They Saved Hitler’s Brain
https://www.youtube.com/watch?v=IKaH2OrIzrw
Perhaps they should mate “The Brain That Wouldn’t Die” (1962) with “Donovan’s Brain”(1953). Then they would have THE MAN WITH TWO BRAINS, after an injection by “Herbert West-Reanimator” Two crazy brains.