Make the frame cut in a straight section at a 45 degree angle for maximum length of the weld bead, and use fish plates inside and out for reinforcement. That's how we did it on tractor-trailer rigs that carried a lot more stress than a pickup frame. I've shortened and lengthened frames that way with no problems. You should be aware that the springs, axles, and a lot of other items are different on a 3/4 ton so you'll probably encounter a lot of fit and interchange problems.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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