I want to convert my 1972 Coronet from manual to power steering. Do I need to replace the steering shaft with a shorter one? Or can the manual one be compressed far enough? I already have a power steering knuckle with the bigger spline.

When i converted my 70 GTX, i just compressed the column.So say tou can't, but i did.

manual can be compressed. Just lengthened the one that was compressed on my car. Compress to where it needs to be, drill 2 small holes through the shaft and use small diameter nylon screws to keep it in place. This pic shows where mine was compressed relative to the original length.