[Lynn Eades]

My guess 5/8" BSF, but not so sure of the size. Definately BSF though. I think the nut locks down on a washer, which is stopped on the shoulder of the stub. This should let the bellcrank move freely.Take one of the other nuts, to the bolt shop along with the stub it screws onto. The nut will need to be thinned unless you can buy a thin one.
Yes you can make your clevis pins from high tensile bolts. For your carrier you need thin flat washers, a(close and) small split pin hole, and not much meat on the clevis pin, past the split pin hole. there is very little clearance at the bottom end of them.