A 1973 Challenger without a front air dam will hold on until about 110-115. Then the front end starts getting light. Over 120 and it starts really fighting against the air. Or so I’ve been told.
I used to run my '72 Challenger in time trial events, totally agree with the above. I never worried about the hood wiggling at speed,
On the back stretch at Road Atlanta I was pushing 135mph, after I fabricated a simple plywood front spoiler. Don't laugh it really helped!
I had it bolted to some simple brackets and removed it when street driving. I just tried to copy the design used on the Sam Posey Challenger, though mine wasn't as low.
If you refuse to drill holes in your lower valence. I'd lower the front with shorter tires or torsion bar adjustment. and block off as much air as you can that comes through the grille.
At high speeds you won't need as large an opening as you'd think. You can install your blocking panels behind the grille if want to keep a more stock appearance.
I'd start by blocking everything behind the front bumper 'cause that won't show. Your idea to move the grille out is good, but I'd still block off as much as you can behind it.
