I have a set of square port heads "3964291"
they look to be in good shape . can anyone tell me what there off of ,if they are a good set to keep (Ipl;an to build a big block later) ,what are they worth?
thanks!

Those heads were used in 1969.

Passenger car 427/425HP
Chevelle 396/375
427/425
Camaro 396/375
427/425
Corvette 427/435 (L71)
Chevy II 396/375

They are a good set to have around. I am not sure of the value but with all those applications they sould be in demand.