collar on Harmon magnum stoker

Hooking up plumbing for my Harmon magnum stoker, on top of stove is a collar for pipe placement 6 in fits inside of collar tight, should I get a adapter to fit over and would not this be safer as exaust would have a smooth transition into rest of pipe outside dimision is 6 5/8 I assume they make that reducer correct me if I am wrong, or is just putting 6 in pipe inside fine.

pipe inside is fine




Eric

Thanks Eric