Would it be crazy loud if I just ran straight pipes out the back from the headers?

yes - very loud! - to the point of obnoxiousness! And, mufflers cause back pressure - which you actually need to make it run properly. Get header mufflers - then run pipe. I had a 360 truck - headers, mufflers and straight pipe out the back - with God as my witness, my neighbors thanked me when I sold that truck - it was LOUD.