Go ahead and start with the 150lb springs since its not a driver. MM told me to go with 250lb, and I did. They are quite stiff, and the car handles well even with the sway bar disconnected. I wish I had gotten 200lb. The springs are cheap and pretty easy to swap out. I think they are around $80 a pair. I had my car weighed and told them the front and rear axle weight, but fox stangs with a 5.0 are gonna be close enough to not matter. You put a big block in there, and you need a heavier spring obviously.
I would prolly go with QA1 if I were you. You already have their CC plates, so I would match all the components from them. Ask if the fit is sloppy between the strut and threaded coilover sleeve that goes over it. Better yet ask someone who has them already. I think they make some 2-way adj. struts too. One thing that sucks about my Tokicos ( besides the high price) is they only have five settings and you cant adj. rebound and compression seperately. Good luck to you.
