Do you have anything passing through the Vband or are they just mating flat to flat? For a Vband to work correctly it needs to have a section of tubing passing through it, so when they are together that section enters the opposite flange. What seasl them is that section passing through heating and expanding into the to opposing flange, not the flat surfaces of the V-band. Most companies/people dont use them correctly and they leak.
headers look good. I think we trained (SCT) the guy who owns that shop out in california when he had a different shop (I think it was DPR maybe?) But his mustang looks familure.