I've got 3 99-04 SD 60s in my garage right now, along with my old GM kingpin 60. The vertical spacing between the ball joints / kingpin / trunnions is exponentially what dictates strength in bending. A perfect side load will induce shear and then the diameter of the joint will be the deciding factor for strength.
The newer ball joint 60s have giant ball joints that are further apart than the old kingpins. The inner Cs and knuckles are also way bigger. These newer axles are absolutely designed for heavier trucks with heavier loads and bigger engines.
The spring spacing is 37.5" which kind of sucks and the 8x170mm bolt pattern is not my favorite, although the wheels are common. There are several manufacturers selling 8x6.5" unit bearings that are set up for bigger stub shafts. You can also redrill the stock bearings. There's plenty of metal there, and they are hub centric anyway.
It doesn't get easier than swapping another GM axle, but the new 60s are stronger in certain areas. The only thing I don't care for is the wheel bearing spacing is very close together, which makes the unit bearing weaker as an assembly. The distance between inner/outer bearing is what gives the strength just like the spacing for ball joints. I've got a couple extra unit bearing floating around that I'm going to dismantle and investigate the possibility of rebuilding as soon as I get my new shop online and build some new fixtures.
There's an 80+ page thread on pirate regarding these new Ford axles. The 05-12(?) Models are more desirable due to have bigger wheel bearings and being set up for radius arms already. You can remove about half of the drivers side spring perch on the 99-04 units however, and that will give you about 2.5"-3" of tube to weld to for a link mount. A guy local to me has also built several units for Ultra4 cars with mounts welded to the center section without issues. If you are still running leaves, you'll need to outboard your springs, which is not ideal on the torsionally shitty frames that the square bodies had. You'll probably want to incorporate the forward spring mount in to a bumper/cross member that is a very rigid beam. The rear will be tricky due to the transmission being in the way. I had dozens of issues with my frame failing back there and toward the engine mounts with stock spacing running a very soft leaf setup and rock crawling.
The turning radius on those knuckles sucks which you'll probably be stuck with if you have a leaf spring in the way.
