[Roger Bolick]

Several driveshaft places all quoted me 3/4" out, i.e. push the yoke in all the way, mark it, measure 3/4" more, place it at this mark, then measure the Center Line of the U-joint to the Center Line of the rear U-joint at ride height. If there is a flange at the rear, use the face. (FYI, it does take most of the 3/4" slide to remove the rear caps, so a 1" gap might work better if you're not accurate in your measurements)

One place said the splines had to be engaged at least 2/3 of their length at level to allow for the rearward movement.... which I'd guess means at least 1/2 of the splines need to be engaged at worse case to avoid any issues. (toploader yoke has 5" of splines for a thin wall tube 1.6825 OD, 50% would be 2 1/2" of spline engagement minimum, seems reasonable)

In an IRS car, both the trans and diff are mounted solid to the frame, so the only movement is the bending of the frame and movement of the diff in its mounts and the engine rotation, so I have trouble envisoning this being more than 1/8" total. The 3/4" will allow movement to remove the rear caps. Based on the above numbers, even a 2 1/2" gap would allow the minimum of engagement! All this work and you end up with 12" between U-joint centerlines, best to verify this due to all different combos.