The short answer is any car ever made with power steering since NOTHING will bolt in place of your 51 box. Your 51 box, mast jacket(column) and steering shaft are all one piece and no p/s was ever offered like that. The two easiest ways to go are the mid 80's Toyota 4wd pickup power box or the tie rod assist cylinder. The Toyota will require cutting your column, installing a bearing and a bottom of the column to floor mount. It is the best set up. The tie rod assist cylinder just requires replacing the drag link with the p/s one with the control valve. It also takes a bracket on the frame (or axle I beam) for one end of the assist cylinder and a bracket on the tie rod for the other end. Steering column remains unchanged. The easiest p/s pump setup is the 50's GM generator with a p/s pump on the rear of it. Neither way is a saturday morning twist a few wrenches and drink a few beers procedure. If you don't cut, weld, design, and fabricate then stay with the manual steering, take it to a GOOD shop, or trade it for a truck that came with power steering.

Evan