What do those 1/2-18 bolts screw into. If they're screwing into the body of the shock absorber, then they're probably specialty bolts or plugs. If they're just attaching the shocks together with a nut on them, just use standard 1/2" bolts, either 13 tpi or 20 tpi.
Are you SURE they're 18 tpi? Did you use a thread pitch gauge? Not disputing you, but I'm not seeing anything except 13 tpi or 20 tpi 1/2 inch bolts in the 1929-1954 parts listing.
