So, after 100,000 miles on my shorty, bolt in headers, I began having loud exhaust noise coming from the driver's side bank while driving. Could not see/here leak while idle ing on hoist, wound up buying a replacement set of short headers (I made the mistake of scrapping my stock manifolds ) and replaced the driver's side header. Found that my SS header was cracked right above the weld on collector on the side nearest the engine. 3 hours later, new header was in and exhaust noise was gone. Drove truck for 2 weeks, and suddenly started hearing even louder exhaust noise coming from passenger side. Since I had not replaced the passenger header yet, I pulled in to the shop again and replaced that header. Found the front tube had completely cracked through about 3" from head. Dandy. Swapped new header in, hated myself for another 3 hours and have now officially vowed to never put shorty headers on a 5.4L ever again!

I did get 100,000 miles out of a set of aftermarket shorty Stainless Steel headers, but replacing broken exhaust studs is still much easier than screwing with shorty headers.

 

The reason short hedders don't offer much improvement is that the stock system is no restriction to gasflow at the 'usual' engine speeds.
The motor has to be revved in the upper ranges to see any benifit from reduced flow restriction.
Long tubes use acoustic tuning more than a restriction reduction due to the length.
Also the diameter has to be kept small to keep gas speed high so better cylinder evacuation is accomplished.
When this is done the incoming charge is not contaminated by leftover exhaust as much and leaving room for more new fresh charge to make power.
At the lower RPM the acustic tuning lengths get physically longer.
Bottom line is there are two forces at work in an exhaust system.
1 the gasflow speed (diameter controlled) and
2 the acustic reflections from cylinder pulsing (pipe lengths).
Same principle as a pipe organ has different size pipes and lengths per the tones to the airflowing through the pipes.