Go F150 springs. I vote those all the way. They are cheap and mine flex really well. I moved my front spring hanger (for the rear axle) forward about 5 3/4 inch and I am using the stock F150 shackles and also some 6" eye to eye shackles. I know you want the ones out of late 80s early 90s F150s and they are about 53-54 inches long. Mine are 53" with no weight on them. I got mine for free because it was a straight trade for some 3rd gen pickup rear springs but at the Pick n pull theyre like $40.
