I always weld them. My experience with them is they’re rarely an exact fit, which can result in leakage if they’re clamped. If it fits well clamp it, but more than likely i’d plan on having to weld it. It’s not a HIGH stress joint, but it does move and requires a good connection, or it can work itself loose if it’s clamped.
Often times i have to trim the ends to shorten the whole thing for a good fit, which wouldn’t allow much room for a clamp.
Check your motor mounts while you’re there. Soft or broken mounts will eat a new flex pipe in no time.
Easy check is to have the trans in park or first gear, parked on a level surface. Hood up, grab ahold of the radiator support and rock the car forward and back several times with all you’ve got while observing the motor.
You should see an inch or three of movement, but. no lifting or bumping noises. if you’re not sure how they feel from the test, have a helper move it while you eyeball each mount.