wheel stud size rear axle 8 3/4?

Have a inside tire rub issue that requires me to add a 5/16 spacer on rear axle. Does anyone know the orig equipment or stock stud length and knurl dia. for the 8 3/4 on a 71 Duster 340 car with disc front and drum rear (if that Matters)? I need to find a stud approx 1/4 to 1/2 longer. Anyone selling them? Thanks