I might be incorrect with this but it's your LOWER control arms that should be just about level if not a degree or two pointing down, which mine are, checking my uppers after reading this post, they are nearly parallel to the lowers with just a slight downward angle. I have Heidt's Carrera coilovers which are dimensionally equivalent to a stock Mustang II with a 302. I have 500# springs with a 460 which is way too stiff. Although it isn't so bad as to not drive the car for last 13 years. When summer arrives here in cactus country and we lay up our cars for awhile I may put in different springs. I would assume a taller or shorter spring would change your upper arm dimension?