But you don't have a C8.25.

Does anyone know if the angle between the spring perch and the pinion is the same on the C8.25 as it is on a Dana?

Based on some pictures I have seen, I suspect it isn't.

Well, first, no one has asked, and we can't tell from the pictures. Does the drive shaft have a CV joint at the top?

If it is a CV drive shaft, then you make the yolk at the bottom point straight at the CV joint.

The it's not a CV DS then you make the angle of the yolk at the diff match the yolk at the t-case. Then check that the DS angle is within spec for angle.

I'd say you will start to feel vibes if you are more than ~2 degrees off. They will be more noticeable than aggressive tires at about 4 degrees? If you are too far out for shims then you can cut off the perches, get new ones, and weld them on at the right angle.

Yes, I have a CV DS. My problem is that it looks like I need an unreasonable amount of shimming to get the bottom yolk inline.

Then you really need to cut off the perches and re-index them to the proper angle.

One trick you can use is to measure ride height, take off the rear leaves and put the body at ride height. Weld a jig together that:

Has a plate on each perch that indexes with the same hole the leaf spring uses.
Has a vertical bar to get you high enough to clear the diff
Has a horizontal bar to get you to the other side
Has a clear mark somewhere around the diff so you can center it perfectly again

Now that you have your jig for width and to make sure that both sides are welded on level to each other, cut off the current ones and grind the axle clean of any remnant of those perches.

Now roll the axle into place under the car and rotate until correct alignment to the DS. Put the new perches on your jig and tack weld in place. Check, double check, put the leafs back in and check, then finish weld and re-install.