It is discouraging that it is nearly impossible to get a straight answer on what is illegal and what is not. Especially considering a huge organization with hundreds of people with hundreds of computers and millions of dollars can't get it together as well as some 18 year old kid in high school in Iowa that wants to legally put a new part on his rifle.
The problem is you asked the wrong question, used the wrong terminology, and asked about a legal term without a simple short answer.
If the question you asked was, "Can I have ___ while I have ____, here is what I want to do." The answer would be much easier because you would not need to be given answers to extra questions you asked.
No, you don't have a tax stamp yet, and possession of parts intended for creation of an NFA weapon even if you are planning to wait until legal to assemble it is possession of an NFA weapon.*
*with exceptions
Constructive intent, and constructive possession have nothing to do with building anything, guns or otherwise. Yet you asked about those terms and how they apply to gun ownership, further confusing the clear answer you wanted.
They are legal terms that apply to ownership or intent to a range of things under law and you are simply mislead by the "constructive" word in them.
You should not own parts that can allow you to assemble an illegal weapon unless they also allow you to assemble a legal weapon, and only if that is your intent.
The ATF is simply a regulatory agency, and you can't get a straight answer from them that will hold up in court on some issues.
Checking with them on a specific issue can be useful for business practices or as a short cut to current interpretation by the people that enforce the law, yet it is not a sure way to actualy understand the law.
There is enough grey area that the interpretation can change with the political climate and thier current budget.
So on many legal issues there is no clear answer, some things are legal and illegal. The akins accelerator is a good example. It was "legal", the ATF stated it was legal, and then it was illegal. No law changed, the ATF interpretation did.
Yet for it to have been determined illegal now, since no law has changed, that means it was always a machinegun and therefore always illegal possession of a machinegun, even when the agency interpreting the law said it wasn't. Legaly they do not make law, they just enforce it (in reality they do make law, since there is dozens of interpretations that can hold up in court).
So had they said it was legal, then arrested people for it without first declaring them illegal, that would still be perfectly legal, because the law never changed, so they were never legal from the start. In fact I don't know what the statute of limitations is for prosecution, but I would imagine the list of pruchasers could be charged and sent to prison for illegal possession of an unregistered machinegun if the ATF decided to do so.
They don't have the authority to just declare someone immune from the law, so be saying it was legal the illegal status of it technicaly and legaly would not have changed.
So things can get quite confusing.
They don't legaly create law, so thier current interpretation of law is and always has been law (at least since the date of the law it is based on) since they do not really have legal authority to create new law. So if you do something considered legal, and then it was declared illegal later, technicaly it would have always been illegal because the law never changed, just the ATF's clearer understanding of it would have.
Owning any item, legal or illegal, even if you store it miles away at a family member's home or a parked vehicle or a in a storage unit is still constructive possession of that item, whether it is a dog, a computer, or a gun part. Constructive possession is not a criminal offense, it is a type of possession. Constructive possession of something illegal to have is illegal constructive possession.
Since having the parts to build a weapon that is illegal with no legal use for them remains illegal, owning those parts stored in different places would still be "constructive possesson" no different than having them in front of you, not because "constructive possession" means you can consruct anything, but because you technicaly still possess an NFA weapon illegaly.
So people see the terms "constructive possession" a lot in cases involving illegal firearms and get confused about what it means. It just means you possessed something that was not present, and was like in your example stored at a family member's home.
If you possess the items which you can only use to make an illegal item you could be charged. Yet if you have an AR-15 that stock can also be used on, you have a valid legal purpose, until of course you mention you have it for use with your pistol once you got your stamp. That just was admission to illegal possession based soley on intent. You can have legal spare parts for legal weapons, millions do. Unless of course the ATF decides they are to build something which is an illegal use for them, then they are illegal.
If you are planning to make a legal SBR you should get the tax stamp ASAP to be legal.
You cannot get everything ready for the project because by being ready you will have broken the law. That is your question, and that is the answer. There is ways around the law by having the items in question legaly for a legal purpose, but because your intent is to use them on a weapon which you don't have a stamp for, even if your intent is to wait until you do, it still is illegal to own that stock.
So possession of it can be both legal and illegal at the same time. Possession even with intent to wait until you have a tax stamp to use it on the pistol is illegal.
Possession for an AR-15 you have that it fits on, and at a later date deciding one of the stocks you have for your AR-15s would be nice on your pistol after you have the tax stamp is legal.
It is all intent. Since your clear intent is to purchase and own it for eventual use with the pistol, ownership of it in any way, even miles away at a family members house is illegal constructive possession of a SBR (once again constructive possession has nothing to do with construction.)