I agree with Jerry. The taper of the frame rail can be adjusted now allowing you to put the hairpins anywhere. I'm kind of new to this as well but from what I can tell, attaching the hairpins under the frame would raise the vehicle. Tack weld frame rails in place, place hair pins with king pins at their 5 to 7 degrees, tack weld (heavy tacks) spreader bar, and tack weld spring perch mount to spreader bar. Then if was me, with weight of car, I'd slowly lower jack until spring is under load. I would however leave the jack under it in case one of your tack welds don't hold. Then check ride height and caster angle and you'll know if you need adjust kick up or hairpin placement. Hopefully spring rate will be close to what you need in your setup.

Guys, typically the hairpins should be parallel to ground more or less shouldnt they?

David