Now that the 700R4 is out I was able to take some good measurements. Both the 700R4 and the 6L80 are 26 7/8" long from forward mounting surface to the rear mounting surface of the 4WD adapter. As far as the mounting holes for the transmission mount, the distance from the center of those holes to the rear surface of the 4WD adapter is 2.5" on the 700R4 and 1.5" on the 6L80. So, the transmission crossmember will need to be moved 1" rearward. Pretty easy since it just bolts in place with 4 bolts. 4 new holes in the frame and all will be good. The driveshafts and TC shift linkage should line up perfectly. The shift shaft on the 6L80 is about an inch?? further rear than the 700R4. I think I'll be switching to cable linkage--I already have the IdidIt kit-- but I'm going to leave the rod linkage in place to see if it works. Even if it does, I'll probably need to switch to cable to make room for the exhaust.
Edited to add: OK, I measured the shifting shafts. From the front mating surface of the transmission to the center of the shifting shaft: 700R4 9"; 6L80 10.75"