Take care with identifying the threads in the bottom of the gearbox… Earlier, vintage and prewar bikes are 3/8″ Cycle Thread, @ 26TPI, and later bikes are 3/8″ BSF. Many have been damaged by unwitting folk trying to screw in the bolt or stud with the wrong thread !
Ken Lack has supplied the needle-roller thrust release conversion for many years, he should be able to tell you the thickness of the assembly, which is actually not critical, as if you run out of clearance on the three thrust pins you can adjust them, or if you have the solid 3/8″ diameter non-adjustable pins you will have to fit a set of longer or shorter pins. These can be made quite easily out of a length of 3/8″ Silver Steel . Just make sure that you make them the same length to within a thou’ or so, and put a radius of about ten thou” on both ends so that they don’t “dig in”. To harden them, heat to Cherry Red, and drop into some waste oil or Paraffin (Kerosene)…. A job that should be done outside for safety’s sake !