Header and catalytic converter connection

Just recently installed long tube headers which have a fixed three bolt flange on the collector. The high flow catalytic converters I am trying to install have a flared end with a free rotating flange with a slightly larger three bolt pattern although the pipes themselves are both 2-1/2". What is the best way to make this connection? Can I run a flange reducer in the flared end of the catalytic converter? If so is a seal or gasket required? Does this have to be welded?