Well I know long tubes would be the best option but my car is going to be lowered with H&R Super Sport springs so I don't know how well long tubes would clear. That said what is the power difference between the three header options?
Personally either go shorty, or longtube. The power diffrence between shorty and equal length will honestly be like 3-4hp. Shortys are way easier to install too you can install both sides in like a hour and a half to two hours. Equal-length do look nice. Im speaking from experience as i have had both on my past mustangs and I went shorty on my current mustang. Next time I think id go longtube though, just gotta watch the clearence if you have tall speed bumps then it could be a problem also your springs are fairly low.
MM&FF had a write-up awhile ago on this EXACT subject, and it was on a 5L too.
The general result was that each header made the within 2-5 PEAK tq/hp of each other, and at the same rpm. However, the LT's at some points in the lower rpms held a 20wtq advantage over the shorty's, and a 15wtq advantage over the equal lengths. So for peak power they wont make much of a difference either way, but LT's make a much bigger difference for daily driving.
Personally i am going with the shorts. The long tubes are more for torque, while the shorts will help to move that powerband up into the high rpm's. Also, my car is bone stock besides the ford racing handling kit. I love it and would suggest it to anyone. But my cats drag over speed bumps and driveways so.... i dont think i could run long tubes