Header Collector Connection

[Skipper]

I have recently purchased a MidStates Cobra and I'm having after-fire in the exhaust collectlr when shifting up. I've looked at the pipes and the collector slips over the headers and is loose. How do I get a better seal on the connection point of the header and collector pipe? I think this is part of the problem, in that I feel it's running a bit rich and the unburned gas in the collect explodes with the pressure drop when it shift. Is this the problem that I'm having, or can someone shed so light on what my problem is and how to correct this. It's a bit embarrassing to be driving a cobra, knowing it catches lots of attention and shift up and the car pops. I'll also take any suggestions on how to better adjust the Holley Carb, 4 barrel, double pumper, I think it's a 950 CFM.