I think it should be a straight replacement. An SM420 was a factory option for the GMC so it should go right in. My experience is with Chevies, though. With them, the 3-speed 1/2-ton had a 9 or 9 1/8" clutch. All other models have a 10 3/4" clutch, so that's something to consider. Do you have a Shop Manual for the GMC? I recommend you have one - very helpful. I think the extension for the torque-tube drive shaft is at the rear of your longbed Jimmy 1/2 ton. And I don't think the length of the trans are different, so the ball/socket joint on tail of trans should not be a problem. Of course the 4-speed trucks have a large removable floor panel/cover for the trans. You'll have to work around that and make a hole in the floor for the shifter. Good Luck. :p

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