My 68 Barracuda was orignally manual steering. I switched to Power Steering and it was one of the best upgrades I have made to the car, on par with the swap from a \6 to the 360.
Manual steering has a ratio of like 24:1 verse 16:1 of power. The swap to power took a slow responding hand full not to mention the slow speed thrashing in a parking lot and turned it into a nibble and quick handling performer.
I will agree that the power steering on mopars from this vintage is way over boosted but there are several things you can do to correct it. The most expensive is to swap in a rebuilt box from Firm Feel or Steer N Gear, these can be had in several levels of firmness. The cheapest if you have a pump with the pressure port on the back is to remove the pressure fitting and place a 0.040" thick washer under the cap of the pressure relief valve to lower the pump pressure (a slant \6 specific pump already has this done). The last is the alignment specs. The factory power spec for caster is +.75, you want to get as much + caster as can be dialed in while maintaining camber. Modern car will have +3 degrees of more. The additional caster will help the car track better (especially if you are using radial tires) and increases the effort to turn the wheels off center. To get to the +3 degree range may require the use of the Moog offset upper control arm bushings but the first thing would be a trip to your local front end shop, one that will take input on how you want it aligned an not just do what the book says.
There is a reason that virtually all new cars have power steering, even ones that are smaller and lighter than an A-body.