Those can't be Z33 rear uprights. They don't even remotely look like that and don't remotely have the same suspension set up as a S chassis car.
Z33:
vs
S15:
You can see the same hump in the above picture that the modified spindle has on it. They just cut/welded on the stock S-chassis spindles.
This is what I want to do:
Which is another S15 time attack car, they are just using Z32 uprights.