I`m not sure if you can find a stronger nylon bolt.
I use to make my own shear bolts for aircraft tow bars by drilling a hole down the center of the bolt. Making it easier to snap if overloaded. but even if you did this to a 1/4 inch bolt, it would still be stronger than the planes undercarriage structure and would result in more damage than it`s worth.
If your mounting blocks are big enough, you could drill and tap the holes up to the next nylon bolt sizes. after tapping, add a few drops of CA to the threads to harden them. Don`t install the bolts till the CA dry`s LOL.