I would check the width flange to flange on the axles......if the spare diff is a 70, the 5 lug axles should swap in. The 67-69 diffs are 1 1/2" narrower. If you have the older diff, yes you can swap the 3.08 ring /pinion & carrier into your diff. You just need to check backlash before tearing the 3.08 diff down, & use the pinion shim with the 3.08 gears. After you are all set up , run the pattern with marking compound to be sure the pinion depth/backlash is correct. 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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