Stove Pipe Installation question

I'm installing ICC Excel Ultrablack double wall and Excel Class A chimney. My question is regarding the connection of the Ultrablack to the flue extension. I have the 10x10 square ceiling support installed and the class A installed with the flue extension attached and protruding through the ceiling support. I'll be connecting an 18" slip section to the flue extension. The slip section has a crimped end that goes up and fits over the flue extension. The issue is that the first 1" or so of that crimped section is single wall, then it transitions to double wall (ID 6 1/2" for first 1" then 6" after that). The flue extension is 6" OD, which means for the first 1" or so of the slip section there is a 1/4" gap between the pipe and the flue extension. ICC has advised me that I should screw through that 1" single wall area into the flue extension and not further down through the double wall area. My concern is that the screw will try to draw the pipe in to the flue extension due to the 1/4" gap and bend/deform the pipe end creating gaps. Do I just tighten the screw enough so that it doesn't start to draw the pipe edge in? What is the issue with screwing further down into the double wall area?

That's great advice, thanks!