Whether you shorten it or not you're getting unnecessary advice on welding one of THESE frames. The frame under your truck is made from mild steel. It is nothing more and nothing less so as long as your weld is done correctly, complete penetration, no undercutting, etc., you can cut it square, run your weld and be done with it.
Take it from someone who TIG welded for 4 years on automotive parts (including shortening and lengthening frames) and 6 years on jet engine parts for Eastern Airlines, a properly executed weld is actually stronger that the parent metal so as long as it's done right, there is no need for angle or Z cuts nor is there any reason for additional plates. Your greatest concern is that you make your cuts precise and line things up properly.
On the other hand, welding exotic steel alloys and/or heat treated metals is an entirely different story.
