It usually starts around 80 minutes of running...
my go-to suggestion for foot cramps is shoes with a wider toe box. if your foot is being squeezed just a little bit you might not notice it over a short distance, but after a while your foot starts to object, and the pain is a message that you need to pay attention to what's going on. since your pain starts after 80 minutes, that agrees with the shoe-fit scenario.
however, 80 minutes also agrees with other possibilities. it could be either the depletion of something such as fuel, an enzyme, or a metabolite. or it can be a muscle recruitment issue. after 80 minutes of running your lower leg muscles have been through several cycles of recruitment and release and whichever combination you have active at that point is not working well together. I somehow doubt this could occur in both legs at the same time so I'm kind of dubious about this, but it remains a possibility.
assuming this is the cause, the recruitment issue can be addressed by ensuring that you are hitting all the right paces regularly in your training, because, in so far as you can do anything about it, recruitment is essentially a neuro-muscular issue and you need to have the right pathways fully functioning to get the best results. the pathways can only be stimulated and activated by running at the correct paces in training, and you need to be doing enough of it for the event you are racing. only you can determine whether you are doing that. so let's assume that you are doing sensible, broad-spectrum, multi-pace training, and it isn't muscle recruitment.
you mention salt, but other experiments with the various depletion options are not covered very well in your post so, assuming you have considered normal dietary deficiencies and are getting a wide range of fruit and vegetables, fats, proteins etc. in your diet, I would just mention the usual runner's standy by suggestions of gels, raw eggs, baking powder, etc., etc.
that just leaves functionality. you have a structure, fascia or communication problem in your feet, which usually implies that you need a broom stick or relay baton, or similar. a rolling pin will do. lay this on the floor, stand up and put your bare foot on top of the baton and roll the full length of your foot over the baton. do not press hard to start with, be gentle, but roll the full length of your foot, front to back and back to front, over the baton four or five times. right up to the heel and right up to the balls of your toes, the FULL length of the foot. then do the other foot. if you feel any grittiness or lumpiness in the sole of your foot, or you hear any crunching sounds, this means you are on the right track. give yourself a couple of days rest, then do it again. gradually, and I mean gradually, work up to doing it 8 - 10 times per session, to doing 2 - 3 sessions per day, and to pressing harder as you roll.
if that doesn't fix it you probably just have bone cancer and need a foot transplant.
cheers.