Hmmmm ...
The rear frame was definitely built for ladder bars. There is no room under the frame for the upper bars, and even the lower bars would have to be short. Too short. (Shorter bars make the pinion angle change more drastically. Longer bars = less change).
You can install the 4-link in the same place the ladder bars are now.
This isn't optimum, by any means, but it would be a significant improvement over the ladder bars you currently have, which go into bind almost the instant the car experiences any body roll.
I would package them as close to the frame rail as possible, say 1" space between the front outer 4-link bracket & the inside of the frame. (Just enough for a nut & wrench) That will position the 4-link around 23" center-to-center.
If you were building a car, you wouldn't do it this way. If you ever decide to re-do your rear frame, you would want to design it so the 4-link goes under the frame rails. You can look at Chris Alston's Chassisworks rear subframes to visually see this. Here is a link, in case you haven't already seen them:
http://www.cachassisworks.com/c-491-...or-ii-3x2.aspx
(For that matter, if you were to re-do this car as a pro-touring car, you wouldn't do it like this at all.)
To help the rest of your package come together, I have some tips:
1. Use "misalignment spacers" on both sides of all 4 rod ends. This is key to allowing more articulation without bind. The 4-link can still bind. It just has more range before it does.
2. Place the shocks & springs as far outboard as possible. I would push the packaging here. If you use coils overs, mount the top of the shock on the crossmember & the bottom mount on a long, sturdy bracket (with angle brace to housing) on the rear end housing. Angle the top of the shock inward at 10 degrees & push both mounts as far outboard as possible (keeping 1/2" clearance between the spring & frame). This width of shock & spring will add stability & control, as opposed to mounting them narrower.
3. I am a fan of panhard bars only when they can be long & adjustable on both ends. In this application, a panhard bar would be on the short side for my tastes, causing excessive sideways movement during travel. It would work ... just not preferred.
I would lean towards a watts link to keep the rear end centered. I always make mine adjustable (for roll center tuning), and even then, you need to get the roll center "in the ballpark." If you're concerned about body roll, you may want to mount the roll center in the higher range. If you were Autocrossing (doubtful?) you may want it in the lower range.
Best wishes !
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
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