If the filter canister is attached to the engine, hard lines are OK. Mount the filter onto the firewall or the inner fender, and flex lines are necessary. Most canister filters were dealer-installed options, so flex lines gave them a little bit of wiggle room about where to attach the canister- - - -ahead of the carburetor or behind it on the intake manifold, for instance.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!