I got under the car to check it out. I pushed the side of the bumper, and saw the fender clearance reduce on that side. Upon closer inspection, the shackle bushings were allowing the car to move sideways. Definitely not desirable. This affects not only how wide a tire I can fit, but also handling. Look at it like this: when turning into a corner, the rear end has to take a "set" by taking out whatever slack is in it, and loading the suspension sideways to the same amount as the force being transmitted into the tires. Further, bumps or transitions can allow the rear end to loosen, then have to take another set. The handling of the car is significantly decreased for transitions.

There are a number of ways to fix this problem. Some are more elegant than others. Here are some that I considered:

1. New leaf spring & shackle bushings: This is a quick fix. It doesn't completely control lateral movement of the axle, but it does reduce flex to a large degree. There is one main problem however: manufacturers such as Prothane don't sell bushings for the Fairlane. I bought their Mustang bushings anyway. However, my shackle eyes were smaller than those in the Mustang bushing set. To fix that, I ran a bolt through a pair of bushings, clamped a nut down on it, and put the assembly into my drill press. I then used my hand-held grinder, running in the opposite direction, and "turned" the bushings down to the proper size. I used vernier calipers to measure NOT the bushing (they were badly worn, and would likely give innacurately small readings) but the actual hole into which they went. I noticed that the leaf spring end size actually varied from the chassis / frame end size, so be careful about not assuming the sizes are the same if you try this. Also, I recommend that you actually check the bushing in the desired hole as you get close -- it's easier to turn them down further than put material back on. This is a messy process, but it provided me with the bushings I needed, and a custom tight fit. Be sure to use the supplied grease on the bushings, and you may with to spray generously with silicon spray too.
2. The second option, is to add a panhard rod (panhard bar). This provides positive location of the rear end side to side, and is a popular choice for racers due to its light weight and low-complexity design. Basically, it is a bar that runs from a chassis mount on one side (usually the driver's side for street and road race type cars due to it's asymmetrical effect on power load transfer in a turn) and a rod that extends to an axle mount on the other end. It runs parallel to the axle, allowing it to move up & down, but preventing it from moving outside of the arc of the rod. While it can be criticized for being asymmetrical, it only displaces the axle 1/10 of an inch for 2 inches of travel up or down (assuming a 34 inch rod). If you wish to calculate for your own lengths, check out this simple calculator. Though a watts link is one alternative that eliminates this lateral dislocation (discussed below), the simplicity is important to me. Unfortunately, a Fairlane owner cannot just run out and buy one. As a result, I've purchased mounts for either end that I'll fabricate in at some later time. Circle track racing suppliers usually will have the necessary components at relatively inexpensive prices. Check out Speedway Motors and Coleman Racing for those parts if you are thinking of fabricating your own.
3. The other alternative is something more complex, like a watts link. Though it does allow the axle to move up & down, it does so without any lateral displacement. Where weight & complexity isn't an issue, this can be an elegant solution. However, it's too complex for my tastes. My car isn't going to be any racer, and I'm having to fabricate everything myself. No need for me to go this route.

So... I did get the poly bushings turned down, pressed in (I made sure they fit tight), and springs installed again. Actually, while I was at it, I installed lowering blocks that reduced the rear ride height about 3/4". The tires do still rub occassionally, but it's only during large deflection or chassis twist. I've not pushed the car in the corners yet, however, and I suspect those shackles will allow too much movement. I also trimmed the inner fender lips on the rear by using a grinder to shave about 1/16 inch off. It's a cheap added precaution. Overall, I'm happy with it as it allows me to drive the car without undue rubbing. Once I get the panhard bar fabbed in, I'll be able to push the car in the corners with confidence that a mid-corner bump won't send my tires tearing into my rear fenders. Once I get it done, I'll post pictures.