So I'm in a handful of welding engineering classes. Now I'm not gonna pretend to be an expert by any means. But I asked my professor what he thought would be a better cut for shortening a frame. He thought a straight cut would be best, minimise the heat affected zone. And he's not just a desk jockey, he was an aerospace welder himself and an accomplished welding engineering. This guy is pretty damn smart, he has a good handle on every flavor of engineering and the science behind it. Figured I'd pass on what he thought.
Now not to say a fancy cut wouldn't work, it's obviously tried and proven as well.