Try the two adjustable pads on your core support

 

Make sure the bumpers aren't missing, like they are on this one (circled in GREEN), and that the adjustable bumpers (circled in RED) are adjusted high enough that the hood touches them about 1/8 inch before it latches, to keep tension on the front of the hood.

 

Make sure the bumpers aren't missing, like they are on this one (circled in GREEN), and that the adjustable bumpers (circled in RED) are adjusted high enough that the hood touches them about 1/8 inch before it latches, to keep tension on the front of the hood.

Looks like I have all 4 bumpers, adjusted those 2 circled in red a little, the hood certainly feels tight, may need to adjust the passenger side one down slightly as the hood sits higher that side (that's also the side where I have a hinge bolt that needs sorting out). I can grab it and physically shake the hood and it feels secure. Will need a road test to be certain though.

I get the feeling this will be a bit of an iterative process.

 

Yeah, mine was doing that at >90mph. Not good. Adjustments and pads as suggested, make it tight. It takes a little effort to close the hood, but it doesn’t move anymore 🤘

 

My fiberglass hood did that at triple digit speeds. Not sure to the actual efficacy of the core support to hood seal for this purpose, however it has definitely helped with the hood catching air and flailing up and down at speed

 

Hood pins, especially for a glass hood

 

I’ve considered pinning mine. But I can’t (yet) bring myself to drill holes in a factory steel hood

 

I have a similar feeling.....
I'm not planning on installing a hood latch but im going to put above panel locking hood latches on my Anvil cf hood

 

Put crumpled up tin foil on each bumper and close the hood. The tin foil will collapse and you can see how close the hood is to the bumpers.

 

For any of you who don't know if it comes up you won't like it. Had a 56 Fairlane with a 352 Fe in when I was 16 . First time the hood blew up it caved the top in enough that it hit my buddy in the head as he leaned over to adjust the radio . Unwadded it & hauled About 2 weeks later it came up going about 90 , I never let up as it went right on over & off the side of the road. We checked it a few days later ,it was trashed .

Oh the good old days . .

 

The hood latch pin is also adjustable. It may be that it is too loose so when air gets up under the hood it lifts off the hood bumper pads.

 

I had to adjust mine when I swapped hoods from steel to fiberglass. If the OP's hood wasn't swapped, I don't see how it would go out of adjustment though.

 

This is how SAI (Shelby American) back in 1965 addressed this. This applies to both the 1965 an 66 GT350:
The hood pin was placed where the adjustable bumper was. (Red)
The Adjustable Bumper was relocated and replaced to the first fender bolt location. (Blue)
As Woodchuck had mentioned the hood bumpers along the edge of where the fender meets the inner fenders need to be in place. (Green).
Adjusting these bumpers will eliminate any flapping at speed. (see pic)

Cheers
~Earl J