I can only say to search this website for the info on BJ's that fit properly. You do not want to be turning down castings or hogging out cavities that are already thin to begin with! They have got to be out there somewhere......!
Those nuts shouldn't be tight, but because they are staked (they are staked right?) you need to unstake them before they will come loose easily. So what you have is a lock nut on the tapered(?) outside of the nut......holding it in position. Post pictures! 
When you tighten the nut the first time, you only putting perhaps 10ftlbs of torque on them IF even that much. I'd need to dig up the torque specs for a 68-69 Mustang/Cougar to see what this needs to be. It isn't much!!!!
Steve
PS I drilled my spindles for cotter pins and immediately discovered that getting to the pins in the recess of the hubs, was a big PITA! Putting a slight curve in the pins helped installation.......but that's about it! Sorry no pic's.....dark ages before digital.......!