Before you try pounding on the lip with a BFH, make cuts in the lip every 0.25 inches or so along the circumference . Then when you pound the lip inwards, it will give easily and the gaps will close.
I use dthis technique to make an extension ring for a UDS and it worked well. No need for a gasket even! I used a cheap harbor freignt cut-off tool to make the cuts and made cuts about 3/8" deep.
Dr. Mark (STL)
Ph.D. (honorary) Bovine $hitology
A thin line separates paranoia from an acute understanding of reality.