Slammed39 First off I need to tell you I am a welder by trade, and a cheapskate on top of that. The next deal, is if (or when) I do this again, I will change some things I did on mine. Things you learn along the way updated and passed along. Also, 39 was the first year for independent front suspension on Plymouths. They used virtually the same suspension design until the mid 50s.
I installed a rack from an 89-94 Chevy Calivier into my 39 Plymouth coupe. The Calivier rack is a rear steer rack like the 39 Plymouths are, with the inner tie rod connections at the center of the rack. The GM rack looks like a 93-96 Intrepid, except the Intrepid rack is a front steer. (sucks too because I had 2 Intrepid racks here and had to buy a GM rack) Tie rods do interchange between the 2 racks. The center inner tie rod connections allow longer tie rod assemblies that can be made to match the original tie rod setup on the 39.
The rack mounts up just behind the cross member, and is a about an inch below it. The tie rods mount at the front of the unit. If your oil pan sump is directly behind the crossmember, there will probably be a clearance issue to address. My car had a 3.9 with a truck pan (a mopar small block with a truck pan should not be an issue.) I made angle brackets that I welded to the frame rails and the rack bolts to the brackets with the factory style straps and bolts. The ends of the rack extend past the frame about 2" on both sides.
On my car I modified a set of Intrepid tie rods to fit through the available spaces, which required shortening the inner part 2" per side, and adding about a 10 degree bend. Then I modified the 39's steering arms by welding the original tie rod holes shut (the original holes were larger then the Intrepid ends, if I had time I may have been able to find steel or brass bushings to take up the space)and redrill them to the smaller size. I used a die grinder to tapper the holes. That worked, but there is a better way.
Better way: The rack has the inner connections at the center of the rack, and they are at the center of the car. Those connections are steel sleeved rubber bushings held to the rack with a pair of bolts (right side uses 1 bolt, left side uses the 2nd bolt) about 2" apart. The original 39's steering box sits inside the frame rail, and the centered pitman arm is about 6" to the left of the center of the car. Both sides of the tie rods connect to the pitman arm. What I would now do is make a "center link" that mounts to the rack using the 2 bolts and going to the left to pickup the original 39's inner tie rod ends at their original center point. To get that measurement, one would only need to center the steering box and take a measurement to one of the frame rails (mark the measuring point on the frame) before removal, then after installing the rack, center the rack and measure the distance from the rack center to where the steering box center was. Make your center link long enough to locate the inner tie rods at their center, and the rack at its center. Then you can use your original tie rods complete. 39 tie rod ends are the same ones used on the 72-93 1/2 ton Dodge trucks.
I had a power steering rack, at 75 mph, the steering was a bit touchy, a power steering pressure reducer would likely have corrected that issue. Also, the car turning radious was larger by about 5' after the rack install. Normal driving wasn't an issue, but there were times when you knew the circle was a bigger diamiter.
1st picture compares the Calivier rack with tie rods to a 39 steering box and tie rods. Gene