65 Hipo Mustang intake manifold right ?

Based on Bob Mannel's book online, it might be, see page 5C2 5-17.

Depending on when the engine was manufactured. The key date for most engine components to be compared against is the engine assembly date. Engines were manufactured before the cars, often on the order of two weeks or more before the vehicles they went into.

In my notes, the closest intake manifold raw casting date and engine assembly date that I have recorded is 11 days between the raw casting of the intake manifold and the engine assembly date the intake manifold assembly was installed in. About three weeks is a more typical span.

In your case, assuming the engine is the car’s original, find the engine assembly date. It was stamped on a small pad in front of the left hand cylinder head. I the event your cylinder block had its decks milled (a common thing to have happened) during a rebuild, the machinist probably cut off the engine assembly date. In that case the cylinder block assembly date should be stamped on the oil pan rail; which requires removing the oil pan and gasket to gain access to. Sometimes the engine assembly date was the same as the cylinder block assembly date. Engine assembly dates a day or so later than cylinder block assembly date are common.

C6 would indicate 66, no? Would a 66 stamping exist in May of 65?

Yes, C6 intakes were used on late 65 kars, my June built K-Code has C6 intake with casting date of 5e12, C6 intakes cast for 66 model year will have 3 holes drilled an tapped for California smog app. on rear machined boss area.