I just built these forks from a 'relay rack'. (two uprights that had held a service-band radio transmitter.)
The back chains are welded to the forks. The front chains pass through a slot (three drilled holes, then filed) and are secured by a tempered bolt (with nut) that is as long as I could fit side to side under each channel. Lifting stress is borne by the bolt head and nut near the outer edges of the channel to get the stress away from the slot. I considered putting a short length of bar stock under each channel to further spread stress but decided it wasn't needed.
After the photo I added a short piece of angle across each fork as a shield between the sharp cutting edge of the bucket and the front chain.
I used a pair of HF 5400 lb load binders but after getting this set up, I think the 2200 lb binders would have been sufficient.
The forks work great, but the project revealed the first problem I have encountered on this 25 year old tractor in the three years I have owned it: one of the Great Bend Loader curl cylinders had sweated occasionally at the lower seal but this new stress caused more significant leakdown. Friday I pulled the bad curl cylinder and took in for rebuild.