Welcome to Moparts.
Rack & pinion steering has been installed in several 46-48 Plymouths.

If you are retaining the original independent suspension, a rack from a 94-97 Chevy Cavalier works very well. Those GM racks have been used in many other GM cars since the Cavalier. That rack has the inner tie rod ends attached at the center of the rack, rather then out on the ends like most racks. They were available as a power or non-power rack, but the non-power racks are much more difficult to find. A standard Mopar power steering pump will work well if you pickup one of those pressure reducing kits that fit inside the pressure hose, or one of the dial a pressure things. Should find both options on a google search for power steering pressure reducers.

The racks mount up nicely behind the Plymouth front crossmember. You will have to make simple brackets to weld to your frame. The outer edges of the rack match the width of the frame, mounting the rack is pretty simple.

The center tie rod attaching point allows you to make a center link and use your original tie rods if you wish (replacement tie rods are readily available, they are the same as 70s Dodge trucks.) You may also modify the original Cavalier tie rods, or you can modify tie rods from a 93-96 Dodge Intrepid. Making a center link is easier.

You will have to modify your steering column, and you have to locate the special GM Ujoint that attaches to the Cavalier rack. I suggest you try to locate the short GM shaft & joint from a junk yard. They may be available at parts stores by now. The rack is pretty cheap, with a very low core charge if you buy a rebuilt rack from a parts store (shop around).

The rack will have a larger turning radius unless you modify (move the tie rod hole 1" I believe) closer to the spindle the Plymouth steering arms.

I used to have pictures,I'll see if I can find them.

If you venture over to www.jalopyjournal.com and check on the hokey azz (ss) message board, then do a search for "rack & pinion for 46 Plymouth" you should find several threads with pictures and updates. I'm "50dodge4x4" over there and have been involved with several of those threads. Gene