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As long as that K5 Blazer is 2wd, you will have the parts you need. To use the 12 bolt from the Blazer, you will have to weld on trailing arm mounts and a panhard bar mount. Getting the pinion angle correct is the biggest challenge. That differential will be 1.5" wider than your original differential, which is also a truck 12 bolt. The '77 Blazer tank may be a bit big for a shortbed truck. You may have to move the rear crossmember to be able to use it. The typical Blazer tank used on these trucks is the '69-'72 Blazer tank.