The adapter would be easier than coming up with a manual steering shaft. I've had cars with manual steering and,,,,,,,,,,,,,,not a fan. My 1st car, in 1981, was a '70 Barracuda Gran Coupe, 383, 4-speed with quite a few other options but one of them wasn't power steering. Driving down the road it was fine but turning a corner built up the arm muscles, along with being so many turns lock to lock. Being a 4-speed, you can't always use both hands to turn. Turning while sitting still? Um, darn near impossible. The other wasn't too bad as it was a lot lighter a car. It was a '66 Dart 2 dr sedan with exactly one option, the 225 S/6 instead of the std 170. Even sitting still, it wasn't all that hard to turn the wheel. It had smaller, skinnier tires than the Barracuda and also weighed over 1,000 lbs less though. I weighed both of them at one point, Barracuda 3,790 lbs, Dart 2,650. you're car would be somewhere in between.
What I mean is, before you decide, make sure it's what you really want.