Your car is 59 years old and is a unibody.  It squeeks.

Do you have polyurethane shackles or spring eye bushings?  When I fist bought my car it had poly shackles on it.  It felt and sounded like there was a pile of chains on top of the bare gas tank than were banging up and down on every little bump.  One of the best days of my life was the day I replaced those polyurethane shackles with some stockers.  No more noise!  Except for the front spring eye bushings... Those make some creaking, but they do not move near as much as the rear shackles do.

Stock rubber bushings are not really "track friendly", but I am betting the rest of the car is built for the street, so make it as comfortable as you can.

If you don't have poly, then check the wheel bearings.  The first thing I do when I buy any old car is to replace wheel bearings and brakes.  Rear wheel bearings can make some pretty horrific noises if they get dried out.

The next most likely thing is upper control arm bushings.  They are metal to metal and when they get dry, it can make some really loud screaching sounds.  Ditto (but not as bad) for ball joints.
 

Check to insure the U-bolts attaching the axle to the shock plates are nice & tight!
Front....Monte Carlo bar and export bar make a huge dif in the flexing of the front end.
Just a couple
6sally6 

Thanks ms everyone.  I always underestimate how experienced and knowledgeable everyone is, then I leave out details .

So:
Noise is ONLY Rear. (random clunk or creak, crank ect from front end spring binding or whatever). But the Consistent DEep Creaking is the Rear during acceleration.

The car was doing all this Ore New Suspension (1in lowering setup, 5 leaf spring setup and POST. It was identical after it left the shop. 

I don't believe it's Poly, I remember the price tag scaring me, but not certain. Seems irrelevant given the statement prior with Pre/Post.

I will check ubolts, but I did buy a new set yeara ago thinking they could be the culprit. Does anyone have a Torque Spec, with the fine thread I didn't want to over tighten. 

I also have the Shelby torque bars. Was getting wheel hop even with the 5band leafs, was also hoping those were going to eliminate the Deep Loud Creaking

lowercasesteve wrote:

Your car is 59 years old and is a unibody.  It squeeks.

Most of the creaking I get in my car are from my old bones! 

Could the new bushings be poly and just look like rubber?  Aren’t some of them black now?
Are the shock mounts intact where they attach to the frame? No cracks or distortions?
Are the leaf spring attaching points to the frame also intact with no cracks or distortions? I had a front leaf spring mount tear away from the frame and needed a good rewelding once.
If the creaking noises are the same after a suspension replacement I would look at some part of the rear unibody wanting to do its own thing and not be part of the team anymore. A good light in a dark garage with a friend might be able to find split seems, cracked welds or something not attached to where it should be.

Since you replaced all the stuff that normally squeaks and you have the same problem, it may be time to start looking at other things that move when you get into the car.  Maybe you can start by having a helper outside the car try to isolate the location of the sound while you get in/out or bounce the car.  The driveshaft could possibly be the issue (you can disconnect the rear of the driveshaft from the differential pretty easily to see if that helps).

Do the WD-40 regimen.   Spray one area where you think its creaks and drive.  If stops you figured out where the problem is.  Still creaks spray in another area and give it a try.  

My though of the creaking is something in the unibody, a crack or something. I'm not a big fan of the Shelby traction bars. I've never used them or have experience with them. What I don't like is they form a trapezoid with the leaf springs. It's not if but when it put the suspension into a bind and I can't help but to wonder if this is the source