The shackles should work for you GMC Jim. You may need to notch one of the box support rails for clearance, & it wouldnt be a bad idea to check the pinion angle after installation. You can pick up angled shims at your local spring shop to correct the pinion angle if needed. Good luck.....crazyL 
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69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....
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