offset shackles (how big a tire)

Since this thread has popped back up I'll add a few things to clean things up a bit

Not all A-bodies have the same size wheel well. In general, a regular Dart/Scamp/Valiant (swinger, hard top, post, 4 door) has about 1" less space in the wheel well than a Duster/Dart Sport/Demon or Barracuda. So if you have a Dart and want to know what fits, don't take advice with someone that has a Duster/Demon/Dart Sport or Barracuda, because with all things equal they have about 1" more room to the quarters. Even an early Valiant has a higher wheel opening than a Dart does which can change things a little when body roll comes into play.

The 1/2" spring offset maximizes the tire clearance for a stock wheel well regardless of which A-body you're talking about, the frame rails and inner wheel wells are basically the same for clearance. So, if you don't do a mini-tub a 1/2" offset is just as good as a 3" spring relocation, the wheel well is the limiting factor. Obviously with a mini-tub the 3" relocation gets you a LOT more room.

The Dart/Scamp/Valiants can usually fit a 245, some even a 255 with the stock spring locations, and can get up to a 275/60/15 with a 1/2" offset. That depends on body tolerances and on some cars may absolutely take some trimming or rolling on the quarter lip. That's more true of the '67-69 cars than '70 up, the wheel wells didn't change but it seems like the pre-'70 Darts had body tolerances that put them on the tighter side for clearance while most (not all!!!) of the '70 up cars have a smidge more room. Just what I've seen from people that have sent me measurements.

Dusters/Dart Sports/Demons and Barracuda's can usually get a 275 with the springs in the stock locations. With a 1/2" offset they can usually get a 285, and if you trim/roll the quarter lip some can even can get a 295 with the stock wheel wells. I have 295's on my Duster with a 1/2" offset and trimmed quarter lips.

All of this assumes you have the right backspace for your car, and all of it is dependent on the body tolerances on YOUR car. Mopar body tolerances are pretty loose, one car can have a 1/4" more room than another with nothing else being different. I've heard of 1/2" differences between the same body style without any accidents or damage, that's a lot but not unheard of. They're not even all the same from one side of the car to the other, so, if you really want to use the biggest tire that will fit you'd better measure YOUR car and on BOTH sides before you decide on what backspace to use on your rims and what tires to order.