New axle.

I think SS Springs recommend 5-7 degrees
With my shot SS Spring.... that were at 0 degrees, i think i will shoot more towards 7 degrees.

I have been told that the best u-joint life is no less than 2-1/2 degrees and average is 3-1/2 Degrees.

As far as your slider, i seen a video, on one of these thread, that show just how much the slider moved forward under launch.
From what i saw in that video, i would say more to the back then center of the slider.