I think its a good thing that it is only a /6 because the price would start much higher than someone may want to pay. Look up the cost of body panels etc. that you know you need and base your offer off of that.
Do you have the body factory service manual? For a 73 Barracuda, go to chapter 6 page 27 and the instructions for your door and quarter glass adjustments are there. Follow those and you should be good to go.
I'm not sure of the output diameter of the HP manifolds but I did install a full 3" TTI exhaust from my stock HP manifolds to the turn downs at the rear lower valance. No adapters and no fitment issues, just take your time installing. When you order the exhaust from TTI, they ask for the cast...
Make sure the threads are clean and make sure the LocTite is primerless or not. If it's primerless you are ok. If it needs primer, then use it. If it is primerless do not use primer because it makes the primerless LocTite weaker. You really can't put to much on the threads because what isn't...
Hard to really tell in the video but if it is a header leak, you should see some black from the area of the leak. Maybe a leak at the collector gasket?
Did you try the long screwdriver to the ear trick to try and pin point the sound or an automotive stethoscope?