I've always felt a bit silly putting safety wire on old shackles that are hard to turn but I keep doing it.
What end do you refer to .. the threaded right?
You'd need longer threads and I think screw pin shackles have tapered threads. One could drill out the threads and employ a hex cap screw w a cotter pin hole. Then you'd need to re-galvanize too. Putting a flat washer between the nut and the shackle ear would boost security. I think I'll stick w safety wire.
North Western Washington State USA