I am sure this has been discussed numerous times, but I have a 1971 Swinger that I am going to be installing a rear spring offset shackle kit on. I am looking for help with rear wheel size and offset along with the maximum "safe" tire size I will be able to fit without any other mods to the wheel wells or quarter panel. Thanks!

I had a 70 that I ran a 9x28 slick with that setup. I don't remember the wheel ofset but I know it had to be right on.

Radial verses bias matters, make sure to keep the cross section or section width of the tire in mind when figuring.

Will be using a drag radial for sure.

I had 4.5" bolt circle drilled into the stock small pattern flange, and found my stock 4x4 79 warlock 15x8 slots to fit pretty well. Can't recall tires as I had tried a few diff. sizes.

What shackle kit? The 1/2" Dr Diff kit or the 3" relocation kit where you move the springs into the frame?

I have been down here before i did the Mopar 3/4 kit and move the rear back 1 in and was able to fit a 275 60 15 drag radial. They may have fit if I didn't move rear back but frt wheel well was close. After that I did the min tub kit. In my opinion I wish I just did the mini tub since u have to weld and move axle perches might as well weld in frt and rear hangers. U can always move tubs at a later date.

With the Mopar "3/4 inch" offset kit (actually closer to 1/2"), you should be able to fit a 275/60/15. The BS I ended up with in my 68 Dart was 4.75", and didn't have to trim anything, though most folks say 4.5" is ideal. You'll have to measure yours to be sure. Not sure how much different 70-up wheel wells are.

I had 15x8 with 7 backspace or 6.5

Planning on using the Dr. Diff offset kit.

With that u still have to weld perches.

ok, cool.. maybe it would just be worth the extra effort to do the weld in relocation kit.

The 1/2" kit will put the outer edge of the springs in line with the inner wheel well lip. It lets you fit the widest tire possible in the stock wheel wells. The 3" kit doesn't accomplish anything more unless you mini tub the car. Mini tubbing will let you run a tire all the way in to the frame. It just depends on what you want to do. The 1/2" kit is definitely the easier modification. The spring perches will still have to be moved in 1/2" per side. Stock A-body perch centers are 43". You'll need your perches at 42" centers.

True on the 3in kit but if you do it and want at a later date all you need to do is the tubs. I had mine all apart cut the perches etc for the 3/4 kit and 3 years later I took all apart for th 3in kit

The 3 inch kit requires cutting the frame rails and is not easily undone, the 3/4 kit is a few hours to change back and no lasting effect.

What width was the rim?

I am just letting you guys know what happened to me. The 3/4 kit is easy and when I was done I thought I would never be back again. A couple years later back in it lol.

And good information, it really depends on the end use and how far you plan to go. My GTS is mini-tubbed and my Swinger is stock. My brother is doing a 67 notch for a guy and came to look at mine because he was thinking of going the mini-tub route, when I pointed out things like cutting the frame and what you have to do with the trunk hinge supports they abandon the idea. I think the relocation kit will serve the purpose for that build cause it is for a 60+ year old man that just wants a fatter tire under it, 275 was his target. This thread got my attention for that reason to help them.

By the way I would love to see how you addressed the trunk hinge/spring issue cause I aint happy with how mine was done (before I got the car).

I will try to get a pic later

I have the 3/4" kit on my '72 Duster, running a 275/60R15 tire. I found that the front part of the spring is still pretty close to the tire's sidewall, close enough that clamps can't be added to the leaves without interference with the sidewall. It currently has stock/fairly tired springs, so the tires are fairly close to the sheetmetal; I'm changing springs over the winter to raise the rear a little and solve that.
Not sure what your springs or future plans are, but thought I'd pass the info on.
On my Duster racecar with the 3" kit & minitubs, 13x31s fit fine, though the openings are trimmed back at front & rear, which you may not want to do on your car.
You can measure out from the center register of your axle flange to wheel opening to determine max tire OD, but allow for suspension movement (compression).