S13 lower control arms are far too long to use in an AE86, plus the width of the bush end that bolts into crossmember is too wide.
A-Spec Steve has fabricated so many S13 to AE86 control arms and has never had an issue. Crab and myself have been using em for over a year now. CAMs have no issue with fabricated bits, no more than someone's custom made trailing arms for example.
I have never used any form of RCA's in my setup and admittedly I do get a bit of bump steer. To correct this, what I am in the process of doing is redrilling my crossmember so that the lower control arm bolts about 30mm higher than standard. Thus correcting the angle of the lower control arm and correcting bump steer.
To fit S13 you need:
AE86 camber tops (unless you want to redrill your strut tops to fit S13)
Some form of shortened lower control arm (speak to Steve at A-spec)
Longer tie rod ends (I use RA40 corona)
And of course the S13 struts, steeting arms and brakes
It will all bolt up.
My specs are:
Tein HE S13 coilovers (Height, body height and damper adjustable) with 10kg/mm Buddy Club springs
Cusco AE86 camber tops
Cut and modified rose jointed lower control arms
260mm? S13 SR Brakes with Trust pad (fukn sick!)
Standard AE86 castor rods with more thread cut for more castor
Celica RA40 tie rod ends with new ball joints
Oh an also some new rack ends that rock! (Cheers AJPS)
Regarding AE86 coilovers built here by A-Spec and AJPS, I have driven a hachi and know many hachis that have these and they work fukn wicked. And the shocks well match the springs. If I had the cash i'd love to have a well built set of AE86 coilovers, as S13 geometry isnt quite as good.