I marked the down pipe with a straight line down it, then cut it and added a 75mm length of pipe so the down pipe was 25mm longer with 25mm over lap at each end. Pipe is then strong with over lap and no risk of any nasty penetration affecting flow. I used a good mig welder and operator (me)
Make sure that your straight line is still straight and everything will align correctly
Mine has been done 10yrs on my 3dr 4x4 and never given any probs