Measuring from the back of my diff cover to the front of my bumper (which lines up with the rear of the frame), I have 36.5" in stock configuration. There is a crossmember in there. Someone had mentioned moving the axle forward, so more clearance could be created either there or by modifying the bumper. If you were willing to take the frame crossmember out AND create some more space via axle or bumper it should not be terribly hard to fit a 35" tank into a 36.5" spot. For someone unwilling to clear the cross member and create extra space, it's not going to happen.
The front of the K5 tank pretty much lines up with the back of the diff cover, if you managed to reverse the spring arch enough it looks like it would contact. But in stock location it is protected by the bump stops. From centerline of axle tube to rear of frame was 39" on the K5 and 41" on the K10 (had to use CL because the axles are different). Consistent with the 2" difference that I measured earlier.