They are low perf 305 heads.
They will run on your 283 but don;t expect much power.
If you want power froma 305 head on a 283 get some #4416 heads
and home port them 1.94x 1.60 valves help too.
Vortec heads work very well on 283's too.
mill .040" off the decks and install with a thin felpro 1094 gasket.
the 450 casting heads will be lame on any motor including 283's.