Stock 340 Manifolds or TTI Shorties?

I do it all the time, but as a last resort .
I first loosen all the fasteners to the motor mounts, trans mounts, and then jack/pry to get as much clearance as possible, then re-tighten everything .
I often take the header off, and belt sand, or weld as nec, - to change the angle the header leaves the head.
I haven't had to ding a header in years. That's way over a dozen, maybe two.
I install a lotta headers, TTIs years ago, then they had rust issues, been installing only Doug's since 05ish.
P.S. leave the bags on till after install, if poss, then wipe/remove handprints before fire-up.
As for stress on the flange, lol - you'll find that the curb you hit when you mis-judge a driveway exit, - will use the collector as landing gear. lol
Good luck .