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Paul, the 300hp did have the same bottom end as the 250hp (cam and all) but it also had different heads. The casting number ended in 461 and they are identified on the outside by the 2 "camel humps" cast into the front of the head. These are the heads everyone calls "fuellie heads' because they first appeared on Corvettes with F.I.
The entire top end (the breathing part) of the motor is what made the 50hp difference. Carb, intake, heads and exhaust manifolds. Without those heads (bigger valves) you won't see the 50hp increase.
BTW, the iron intake that was on your motor was cast on Dec, 5th 1965 at Flint.