Front coilover conversion size
maybe this information will help....maybe not
not sure how you are gonna fit a 12" in there...kinda limited on space.
I find the QA1 #DS 303 works best on the HDK (coil only) conversion. I do cheat the upper and lower mounts out slightly for coil clearance and you can gain some clearance by modifying / grinding the LCA bump stop
compressed................ 9.5"
extended...................12-5/8"
recommended spring....7"-9"
http://www.summitracing.com/parts/qa1-ds303
good luck with your project
Denny