yo Scott- some suggestions you may want to try your 1968 bird stock spring # is 397910 which any local auto jobber should be able to supply after crossing over part#. In any case I have right in front of me a set of springs I removed from a 69 400 car with is the same. the dimensions are as follows, lenght 17.5", width 4.75" 11 coils with .625 wire. I measured a six cyl. spring and it's approx. the same but measures roughly 9/16 wire. As for the control arms the top is attached to the frame through the pivot rod and the bottom is attached by "U" bolts the distance between is say for example 12", that distant remains constant,if you install a longer spring the lower control arm is forced farther away from the spring saddle {which is part of the frame}pushing the car higher because the wheel is on the road.