Frame or chassis options for A bodies
I would advise that larger bars can have clearance issues with the headers, I don't think you will have any issues if you stay under 1.10" but above that they can be tight.
If you are buying used bars watch out for the clocking of the hex heads as they are all over the map and some can be a pain to get the correct ride height with.
I would also replace the ride height adjuster bolts at the same time as you install stiffer bars as they may be more prone to stripping with stiffer suspension.
If you really start lowering the car with a factory front end you start running out of suspension travel, on bad bumps it will bottom hard on the bump stops.
I would say if you are not racing the car go with the PST 1.03 bars or maybe Firm Feel 1.06s, they should give you about the new Mustang ride quality that gets tossed about.
Good point about the diameter of the torsion bars. I did need to put some dimples in my Doug's headers to clear the 1.12's. But that's also with a spool mount K, and it seems like they have more issues with header clearance to begin with. The earlier biscuit style mounts can also be shimmed to adjust the clearance, which is a no-can-do on the spools. The factory tolerances on these cars are such that some cars would probably be fine, others would need dimples. It's a tight spot, but it's not much different than header clearance is on these cars anyway- depends on your car and your set up. I really like the Doug's but I've come to the conclusion that the TTI shorty's are probably the best way to go for an autoX set up, more clearance for larger torsion bars and more ground clearance than the long tubes.
I've had no issues with the torsion bar adjusters stripping, even with the 1.12" bars, and I haven't bought any new adjusters. Just make sure that they're in good condition (threads not damaged) and don't adjust them with weight on the front end. If the threads look good, I can't see replacing the adjusters with new ones that were made in China, for example.
As far as lowering goes, you just have to match the amount you lower the car with the size of your torsion bars. Larger torsion bars allow less suspension travel than smaller bars, so you can lower the car and reduce the amount of available travel without problems. For example, my 1.12" torsion bars have a 300 lb/in wheel rate. That's almost
3x higher than the stock torsion bars. That means I only need about 1/3 of the available suspension travel that a car with stock torsion bars and ride height has. In my case, I replaced the stock bump stops with .375" tall polyurethane buttons, and have just under an inch of clearance to the frame at ride height. I rarely ever bottom the suspension, it takes a really big pothole and a hard hit to bottom it out. And the stock suspension would bottom in that case as well, the stock progressive bump stops get used more frequently than most people think, even at the stock ride height. Best way I've found to see how often the bumpstops are really touching is to put a small dab of grease on the bump stop and go for a drive. With the stock progressive bumpstops I would wager that most folks are on the bumpstops significantly more than one might think. With poly buttons it's pretty easy to tell when you hit the bumpstops. If you find you're hitting the bumpstops frequently, raise the ride height a little and try again. My car sits pretty low, the K frame is just a touch over 5" off the ground, the tops of the wheel openings on the front fenders are at 25".