Yes, bar length and diameter are determiners of basic spring rate, just as the number of coils and wire diameter and checking height are on coil springs. Lever arm on the control arms is a factor in the mix too.
With all due respect, though, my comments were based on two things. The first was a reply to a gentleman with a torsion-bar equipped Chrysler that lived in logging country that had many rutted roads. He wanted to raise the car in the front with the bar adjustment and in the rear with other means. The reply was that he could raise the car with the front torsion bars, but he would make it uncomfortable to ride in due to the increased ride rate of the torsion bars being twisted as far as they would be. This was in something like Motor Trend and printed in the later 1960s.
When we had had our new '66 Chrysler for a year or so, we took it into the Chrysler dealer and they adjusted the front bars up as they said they were a little low. Sounded good, until we got out on the street and the "jouncing" that didn't get done during the adjustment procedure happened as we drove. The car had a "hot rodder's" nose up attitude and it did ride stiffer than before.
Many Texas DPS cars (of the early 1970s) used to have the nose-down attitude that they probably liked for allegedly better high speed road holding, but the old line Chrysler service manager at our local dealership attributed it to them jumping ditches in the medians. This look might be "in", but it's not the way the cars were built, as illustrated in the sales brochures and service manuals. Even if the car had hd rear springs, it should be level as I mentioned (the "heavy duty suspension" spec in the front end ride height specifications).
A torsion bar might be just an unwound coil spring in principle, but it has significantly different characteristics in other respects that give Chryslers their unique ride quality (even some early 1960s Chevy light duty trucks too!). Shimming a coil spring has no change on its basic spring rate, yet cutting coils will stiffen it as the ride height is lowered somewhat, just as a shorter bar of the same diameter should be stiffer in ride rate. From my experiences, when a Chrysler (straight bar) torsion bar front end is set to factory specs as to ride height, the propensity to bottom out is significantly reduced and the ride rate is firmer than if it's sitting too low. Not to mention the affect the lower ride height has on the great steering geometry of the steering linkage.
If the rocker panels are parallel with the ground, the aerodynamics of the car are not adversely affected. High speed handling, even with a full load in the trunk and a full load of passengers, which might result in a slight nose-up condition, have no adverse affects on the handling of those Chrysler products at normal highway speeds or even extra-legal cruising speeds. Been there, done that. In fact, the Mopar Perf Race Manual recommends that drag race cars have their alignment set with the front end jacked up an inch to compensate for the acceleration mode they normally are in when drag racing. Considering what's behind the front bumper on those Chrysler bodies (i.e., strut mount cross member that acts like an air dam under there), it would take quite a bit of raise past the factory spec ride height to get enough air under the car to adversely affect handling or fuel economy at normal highway speed, I suspect.
Sorry for the length, but these are things I've discovered and observed on my own during the past decades of Chrysler product ownership, driving, wrenching (including adjustint the front torsion bars), and being around them in various venues. I might be something of a purist, but those cars work so fantastic in so many ways when just set to factory specs that it is unreal.
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