I really do not think that the notches are really that important,
yes I notched my rails for my forks, but I can not slide them with out angling them down ward and letting them basically hang from the top rail,
I usually try to get one fork in a notch, but I have never had a fork slide or move when in a using position, and I use my forks to move bales and to move pallets and other, by loading the bales on a trailer and then unloading them off a trailer, I have moved a few hundred bales a year, (once on the trailer and then off the trailer, and then to feed the bales, (big round bales)) so that is three times each, and at a few hundred a year that is a fair amount of use on the forks, not counting moving other items around, never had the forks move with out me moving them even if they were not in a notch,
I my self would not be that worried about the notches, I think mine are on a 6" spacing, out towards the normal pallet size I think I reduced them to 4". but since it is hard to see the forks in front of the tractor, I normally off set them to one side so I can see one fork on pallets.