Would a 428 equipped car be built with the same GT group parts ( springs + shocks ) as a 390 car except for the spacers and a 15/16" sway bar ?
Allot of things play into how a car comes equipped (individual parts) think the first step is comparing like vehicles - same body type, trans, time of year, air conditioner ....
Looking at Buildsheets (don't have any non- Shelby S code nor many non-Shelby R codes) I see a few differences between what appear to be similar cars but that was with just a quick look But those same sort of differences can be found between one 428 equipped 68 and another, looking at 68 Shelbys as examples