Whoever told you shorties/tune net you 20hp is a buffoon. Honestly, if I had shorty headers I wouldn't tell anyone. Ford did an amazing job with their headers. Most shorties flow almost exactly the same as factory ones, some may flow worse. Just say you got them for free if anyone asks why you have them, haha!
Longtubes are a huge increase in flow though. You will absolutely see gains with a LT setup, but you'd be smart to run a tune for them. I won't quote numbers because I'm not sure what exactly you can expect, but you WILL see a significant gain.