IMO collectors should be clip on. If it helps install then by all means use separate flanges. I have two cars that are done this way with no issues sealing the ceners with a 3/8" flange. Our tube chassis and dragster stuff is a common single flange as there are no issues with getting them on but still use slip on collectors.

"I am not ashamed to confess I am ignorant of what I do not know."

"It's never wrong to do the right thing"