Yes, it can be legal to make a silencer. For everything Form-1, from silencer designs that are easily made, to filing forms with the BATF, to 3D modeling. Remember, you must have an approved BATF Form-1 to make a silencer. All NFA laws apply.
Sure the autozoners know what to look for. You must have dealt with a Noob. You see, they have them in drawers in the back and those drawers are (should be) labeled inside the lid with the part number, size (std or metric), and of course a picture for those who do not speak English as the picture is bi-lingual. Besides, everyone knows that you just need to ask for a mid 70's chevy V-8 if its the full-time cashier who is helping you. Even then, the cashier should know how to look for parts too and be able to assist you. BTW, the 3/4" freeze plug will most likely be a little bit bigger because it has to fit tightly and not pop out.