When building a drive shaft for a shackle reversal application, it is important that the drive shaft be built to accommodate the full range of travel through the suspension.
In a standard application (assuming the springs have curvature), with the shackles up front, as the springs compress and if nothing else happened, The drive shaft would need to get shorter. But as the springs compress the shackles swing forward, this if nothing else happened would cause the drive
shaft to increase in length.
As it is, on a normal application these to things work in opposition to each other and you will end up with a relatively stable length on the front drive shaft.
The complication comes about with a shackle reversal that now these two actions will now be working in tandem. As the springs compress, the shackles swing rearward. Now you have simultaneous actions causing the drive to compress substantially more than it would otherwise. This "tandem action" will also cause the drive shaft to lengthen more than it would otherwise under full axle droop.
With a shackle reversal there are other factors to consider also: If the springs are flat the drive shaft will usually compress only minimally but extend a great deal. Longer springs and/or shackles will also effect the
range of travel required on the drive shaft.
On a shackle reversal set-up, I recommend that you measure for length of the shaft at all possible suspension positions. Compress the springs all the way against the bump stops, then at static ride height and jack the vehicle up, support the frame with jack stands to allow full axle droop and measure at this point. It would also be a good Idea to measure the slope of the drive shaft (can be simulated with a string line) relative to the transfer case.
You will also need to factor in things that can't be duplicated in your garage. For example there will be times when you are coming down a hill and get the wrong wheel in a chuck hole and a little frame flexing that will cause the drive shaft to compress more than you could simulate while cycling the suspension. Also there will be times when your climbing a hill and the front suspension drops, at the same time while under power the front end will want to pull away from the vehicle. Additionally while under power the front pinion will dive toward the ground requiring a drive shaft that will extend to a greater length than you have measured for. We can build a drive shaft that will have a range of travel from full compression to full extension of nearly 2 feet. although most vehicles will not be able to utilize this extreme travel because if the axle drops far enough to use up all of this stroke the shaft would be running at so steep of an angle that it would bind up through the "U" joints. Remember the suggestion to measure the relative angle of the drive shaft?
You probably don't want to spend more than you need to for a drive shaft that has more stroke than you need. And you certainly dont want to buy less than you need because it just won't work and would be wasted money.
There are also a few down sides to the drive shafts with greater than 6.5" of stroke. Among them are: Cost in a 1310 two joint shaft ($319 for up to 10.5" of stroke or $369 for up to 22.5" of stroke), Sheer mass. these types (over 6.5" of stroke) of are very heavy and are not recommended for high speed use. Maximum recommended speed will be a function of the length of the drive shaft. Also because the shaft is very heavy, as the splines wear and they will, the drive shaft will create more severe vibrations with less wear in the splines than a more conventional drive shaft.
For these reasons, If the extreme stroke shaft is what you need, I suggest that you remove the drive shaft from the vehicle for normal everyday driving (it won't wear out on the shelf) and install it just prior to your 4 wheeling trip.