The most common combination is a 12" spring with a Qa1 DD (double adjustable) or DS (single adjustable) 501
QA1 DS501 QA1 Proma Star Coilover Shocks | Summit Racing
HDK 64 to 66 A Body Upper Shock Mount Support Hoops
IMO, the RMS (copied by Control Freak and Gerst) upper shock mount bracket that attaches to the K limits the shock and spring length and also forces the spindle further outward (for coil spring clearance). It is not a bash, it is just their design / function.