Stumped and Frustrated with a Holley 670 Hesitation

What's your vacuum at idle? Your power valve is a 6.5 you say. Your PV should be roughly 1/2 your vac gauge number. ie if your vac gauge is reading 13 inches then a 6.5 PV is about right. Something just to check that's easy/quick. Also an easy thing to check is if your PV is blown out. Turn your idle mix screws in. If the car dies your PV is not blown.

I would add too that I'm running a very similar setup to you but with a little bigger cam in the Hughes Whiplash with a 4150 750 DP with a 4.5 PV and 71 primaries. My vac at idle is right around 10-11". If runs phenomenal, pulls like a bear and blows the tires off when you stab it.
So I'd check your vacuum and make sure it corresponds with a 6.5 PV or change it and put those 69's in the primaries and see what happens.

Good luck! Carb tuning is just a lot of "one at a time changes" and adjustments then test. It seems like your just not getting fuel and it's going lean and falling on it's face. Had that issue with some vac secondary carbs before.