How about I un-zombie this thread with something I've been working on for my racecar?
S-chassis rear coilovers on Z32 upright, with a cheap little adapter kit sound good?
Not pictured, there's a slug that gets installed instead of the bushing with 2 holes in it, you then bolt the black bracket onto that, and then bolt in the S-chassis strut bottom into that... aligning the directions of everything is also super simple, and the slug can be installed backwards to adjust the strut position as needed
Most of the solutions I've seen for this involve making either a custom fork for the coilover, or cutting and welding the cast aluminum mount, which introduces it's own problems
I am aware that some S13's are slamming their suspension to a point where every last thread counts, and with this adapter you will lose about 40-45mm of maximum slammage, so it will be up to you to figure out if your setup can accommodate it... S14 or subframe swapped Z31's shouldn't have any issues
The axis of the slug needs to match the strut axis at ride height when looking from the front, the strut can then be attached to the upper tabs by through bolt, which gets tightened at ride height (just like any bushing)
I did investigate making a bracket that mounts the strut in various directions around the Z32 upright's bushing pod, and in every case, there was interference or weakness, this seemed to be the only sensible way
The bracket is 1045 steel, e-coated, the slug will be anodized aluminum, with all zinc plated fasteners and locknuts... the bracket weights about 280 grams, I expect the total kit will add around .5KG to each corner
The additional weight is easily offset for most coilovers, because the z32 fork style bottoms are generally steel, at least the ones you are likely to have modified, whereas the S13 style bottoms are aluminum... after saving ~8lbs according to some threads going with the aluminum upright, you might actually turn out weight neutral