Dremel and cutting disc to weaken the nut, failing that Angle grinder and a steady hand to notch the most accessible side of the nut, then a few smacks with cold chisel to move it on one flat, then repeat. Be prepared to loose the bolt, but this is preferable to sacrificing the rod and cap.
It's a perennial problem finding a thin walled oddball tube or socket that fits. Big journal bolts, being slightly longer, (3.5mm on a set to hand) plus a custom spacer washer under the larger later nut are one solution, raising the nut, as long as the ensemble does not catch on the crankcase or pick up pipe. Machining a bigger chunk out of the cap to accommodate later hex nuts and thin wall socket takes more courage.
Swarfy.