Hello Action,
If I may interject on this one. In an ideal world where the myriad of correct coil springs were reproduced I would whole heartily agree with changing the coil springs. However you're lucky if you can find two different springs applications for the '65-'68 full size where Ford has a listing of about 2 dozen and used them for specific body styles (2 or 4 door, post or non post, convertible, etc, then engine options mattered, so did A/C and P/S). One might think one size fits all and that's not true. Chances are you'll end up with a car that rides to high or too low.
Here's the problem with deviating from the designed ride height. The 3D geometry of the upper and lower control arms isn't a perfect design. There is a small amount of wheel movement (up-down) that's the most stable in caster, camber and toe. Designers build this stable area in the middle of the travel withcidentally matches the specifications from what the car exterior designers wanted. Spring rates are chosen to keep a certain weight front end in the middle of this stable area under normal driving conditions. If you end up higher or lower than specified ride height you can start to enter the part of the wheel arc where either caster, camber or toe start changing drastically and will result in a squirrelly feeling car and depending how far you end up it can be dangerous.
I had a chance to study this first hand with my better half's '96 Impala SS. The previous owner lowered the car by heating the springs, which is a very bad practice. The car already comes 1" lowered from the factory on the Caprice suspension, GM knew this was flirting with the unstable zone, but rather than make changes to the Caprice design, they kept that, lowered the springs but drastically increased the spring rate. This makes those cars ride hard, but it keeps it out of the unstable area. Now heat, soften and lower that spring even more and it was pretty dangerous to drive on the interstate in high speed curves. It was all over the road and no amount of other new parts or front end alignments would fix it. I had to hunt down original springs to fix it properly.
Now I'm having a similar problem on my '68 XL. The previous owner had the front springs, suspension bushings, ball joint, tie rods, etc changed thinking it needed it because it's all over the road, and I mean dangerously all over the road. But here's the problem. Now the front end sits 1.5" taller than it should and it's flirting with the unstable zone. The original problems from what I see is the steering gear is worn out (pitman shaft side movement for sure) and the rear axle bushings are original and deteriorated and the rear axle is moving around under the car. So we still have the original problems plus a new one created with the new springs. Fortunately I have a parts care with the exact same options and so I can use the springs to fix this new problem he created plus then fix the original problems.
To Michael Rosepal: check your ride height, if it falls into specifications in the factory service manual, I would strongly encourage you to keep the original springs and simply upgrade the sway bar. You're not going to throw anything else "out of whack" with a heavy duty sway bar. I bought a Hotchkis and it fits very well and I'm really picky. I mean really picky.
However I would like to verify that you are talking about body roll and not wandering in your lane? Two separate problems. To quote the line from Apollo 13, "work the problem, don't it make worse by guessing". That's very true with an old car as you can go down a very frustrating and expensive rabbit hole extremely fast.
Cheers
They spent a lot of time getting the ride quality right with a fair amount of specialized equipment. Unless the aftermarket supplier has the original Ford drawings for the springs and can use the same process methods there is no realistic way they can make a direct copy.
