So, like this?
Because it's a lot more difficult to brace that properly, and you end up with this:
I would suggest
not limiting yourself to a particular spring. Build around a standard thing, like an XJ front or ZJ spring, so you have a lot of options to play with. I tried V8 ZJ fronts (way too tall), Chevy Camaro rear springs (right height but too soft), and now I'm on the stock ZJ rears which work well.
What kind of shock angle do you run that allows for +5/-18? Why those particular numbers? Sounds a bit excessive IMO.