Axle shim advice

[tr4boy]

Folks,

I'm in the middle of converting my 1954 GMC 100 to power steering. It will still have the straight axle/leaf spring suspension.

The instructions recommend increasing the caster by 4 degrees using a shim between the axle and the spring (fat side of the shim pointing to the rear of the truck). I have the shim and understand (mostly) how to install it, but I'm wondering if I need a corresponding shim between the top of the axle and the bump stop mount to keep the u-bolts perpendicular to the shims (the fat side of this shim would point towards the front of the truck. Am I overthinking it? I'm including a picture of the axle for reference.

Also, assembly manual for the 1954 Chevy 3100 (which I assume is the same as the GMC mostly) shows an axle shim installed. My truck currently has NO shims but was aligned within the past year with no caster problems. So, is the shim only needed to correct a caster problem or is the shim needed all the time to establish the correct caster angle? I'm including a picture from the manual...

Thanks for any advice!

Greg