As far as I know, 64 thru 68 spring perch bolts were not welded, they all had splines as retainers, both 3/8-16 and 3/8-24 bolts. The MPC indicates these bolt part numbers, 373134-S(2), 380042-S(2) and 381792-S2.
I have been told that the bushing was tack welded to the perch on the inside for all HP Mustangs including Shelby. That I have not seen or been able to verify.
Jim