The link I posted earlier, IIRC, has a pic of the frame lengthening and it was done under the cab. The cab actually has six mounting points.
http://www.froadin.com/tim_melton.html
"Tim started with a 1964 J 300 and a 1985 Grand Wagoneer. The J 300 box is shortened 24", and the frame is lengthened and boxed in 8" behind the rear spring mount..16" of the rear cab is all that remains of the J300 cab"
I've got a pic of the frame and will post it when I find it.
*** Actually the frame stretch pic I have is from another project supercab:

I think you could use a narrower cab and chassis rear D70 that would fit inside the wheel wells and then cut the fender flare off and replace it with a modified Thriftside fender and it would be one sharp dually. You might have to move the inside of the wheel well in an inch or two depending on what tires are fit, but it looked to me like you could get by with out doing that. The springs would have to be inboarded, but using the narrower axle would allow you to minimize the width of the thriftside flare actually needed. The following pic is of a custom flare fit to bed using the wider dually axle and I think you could get by with only 1/2 that much extension using the CC axle.
Inboard spring hanger and CC wheel to frame clearance:
http://www.fullsizejeeps.com/jeeps/files/Parts-Junkyards/LRFwdInbSpringHanger.jpg
http://www.fullsizejeeps.com/jeeps/files/Parts-Junkyards/WheelFrameClearance.jpg
