Just by the nature of your statement, "a BM59 clone", you ought to have known it would be prohib.
You guys DO realize that the BM59 was a full auto rifle right from the get-go, right?
Of course, if you try any mod "that will basically turn it into a BM59 clone" you are for sure going to be making a prohib item.
You had to contact the lab to find that out ??
You have been mentioning modifying an M1 Garand receiver to convert it to a BM59 (which infers full auto). What were you thinking?
Did you honestly think their answer would be any different? ;-D
Maybe you were asking the wrong questions to whoever you spoke to at the lab. Certainly, referring to a BM59 as your end goal is your main big mistake, needless to say.
FYI, There WERE semi-auto only Beretta models made that were/are perfectly legal. Do a bit of research to look them up before you go asking the lab again. Remember that the prohib order is targeting the full-auto aspect of the BM59. Stay away from that and there ought to be no problem.
If you're simply taking about adding miscellaneous stocks, barrels or anything else from a semi-auto only M14 / M1A or M305, there's nothing wrong with that.
There is also nothing on the books against any kind of buttstock, as long as you keep the barrel and OAL within the unrestricted limits. Buttstocks in and of themselves are not controlled items.
Also, having the ability to use an M14 box magazine, in semi-auto only mode, is also not prohib. It may ruin the Garand's capacity exemption for its 8-shot stripper clips, but it is not prohib.
And, if you stay within the unrestricted barrel and OAL length, you can put any buttstock on a Garand or an M305 that you want.
I was advised quite a while ago by a senior person at the lab myself, but I did my homework and asked the right questions. and avoided the wrong references. Our conversation surprisingly included some tips I got for working on the Garand, and ended on good terms, with best wishes from the person on my project. ;-)
Cheers.