The least expensive solution that gives you a pretty good off road suspension is to take the rear stock springs and move them to the front of the vehicle. This will also move the front axle forward 2" to help larger tires clear the firewall, but be sure and check the engagement of the driveshaft splines afterwards.
Then go to a junk yard and get a set of rear leaf springs out of a late 80's to early 90's chevy 1/2 ton pickup. Install those in the rear. You will have to relocate the spring hanger a few inches forward on the frame.
At that point the 4" shackles may be OK as long as the springs do not have negative arch. If the suspension is too high you can shorten the shackles by cutting them off and drilling new holes.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang