I assume the Vega box is cross-steer? Cross steer and split bones /hairpins is the problem. Split bones allow the front axle to "parallelogram" itself [shackle wobble] and will shimmy via the cross steer. Install a panhard bar [which you haven't mentioned]
They are radials I understand...I know I've certainly ran Bias plys low and lower than that...can you run radials that low?
I had a front end shimmy in my Sport Coupe some years ago. Couldn't get the kingpins to wobble, replaced front shock absorbers because one felt a little soft, replaced tie rod ends because one felt a little gritty, replaced 4 bar bushes just because everything was apart and the had about 50,000 miles on them, put a different pair of front wheels on. No change. The front end is a dropped 34 axle with 4bar, panhard bar, disc brakes, cross steer and a small scrub radius. I then decided to change the king pins and bushes anyway. When I removed the cotter pins from the axle I found that the left hand stub axle rocked. The problem was an ovaled king pin boss in the axle. I had the axle bosses re-sized and re-reamed then put it all back together. The test drive showed that this fixed the problem. Phill
One reason the stabilizer works is because it stabilizes flex in a small diameter tie rod. The vast majority of beam or drop axle cars use a wimpy 1" diameter tie rod, and they can easily flex. The shake or death wobble is often caused by the tie rod flexing and that can happen at certain speeds, or when going over bumps as you're traveling down the road. The stabilizer attached to the middle of the tie rod simply keeps it from flexing and shaking. But a better fix is to go to a larger diameter, or thicker wall tie rod that doesn't flex. I use larger 1.25" tie rods and drag links on all my straight axle builds, and have never had front end shake, or death wobble on any of them.
I had death wobble in one of my vehicles. The only thing that tamed it was the steering stabilizer. That had a tie rod, and a drag link made out of 1-1/2" 0.250" wall DOM.
My ‘32 drove amazingly well without the VW style stabilizer . I finally added one and it didn’t seem to change much that I can notice . But after driving a straight axle 4x4 Power Wagon daily . I think a Radio Flyer would drive and ride great .
Here is the VW damper installed on my Logghe car. I'm done racing for this year so I will try it out next outing in the Spring. It has a 6" range of motion.
Ford makes one for I beam pickup trucks available at most autoparts and goes on with 2 clamps that come with it. They work great and go on in 15 Minutes.
The P&J stabilizer fixed my death wobble but I also changed all of the poly bushings on my front end also. Hope U get it fixed soon. It's a shakey situation.............
like many others , i have gone thru most all of cross steer . slution is a good track bar ................ and no i cant spell !
Caster angle can be very critical on the early Fords. 1/4 degree off can cause shimmy problems and running radial tires just adds to the need to get it to it's happy place. I think, from experience, that you may need to reduce you caster angle. 7 degrees sure didn't work on my A V-8. It was horrible. Kept reducing it a little at a time until, suddenly, no more shimmy. I did add a stabilizer after that but doubt it has much effect. Each case is different, however and I don't think any two need the same treatment. The fact that it only does it when decelerating has me puzzled. Speeding up or slowing down should not cause it to shimmy. You may have another problem.
You have to do all of this: ballance and true the rims and tires, have between 3 and 6 Deg of caster, adjust all the play out of the steering box, put new Mopar (big) rod ends on drag link and tie rod, ballance the rear wheels and tires, they can transmit wobble to the front end, add tube shocks, keep the tire pressere about 25 lbs and ad a steering stabilizer.