The more I think about this the more I realize it is the short stroke of the jack that is the issue and not their mounting location. If you have 18” under the jack foot when fully retracted or 6” once you block that space with blocks or the leg extension hole you then have the stroke length of the jack. If it only extends say 13” then it only extends that. The issue is more the long trailer needing that much more jacking to level.
If it were me I would first try blocking up the rear of the truck before unhitching before I tried the jack stand method. Jack stands will work but it is an added step and if on a slope there is some instability pushing downhill on these jackstands as you transition. I think four pieces of 2X8 would work in lots of situation. Two 3’ and two 2’. Back onto the 3’ers and then set the 2’ers on and back up again. For rehitching I would throw some quick marks on the ground, with a marker or spray paint to show exactly where I need these blocks. Just my thoughts, have Had to do height adjusting hooking grain trailers in the field in combine season and we always either blocked or dug in the truck tires.
Of course a shovel and digging the trailer tires down a inch or two would have a greater effect, and samplers, but you would have to watch clearance on the rear and most places won’t allow this.
