I understand your question and, since I don't have a copy of the letter you received, I am not sure what the timeline is exactly for you to make a decision, but I do know that you do not feel confident about your potential choices.
Since this is a 401k, you will have the option of taking a distribution of the entire amount and rolling the 401k into an IRA. This is especially useful if the investment choices of the 401k are no longer to your liking or you are uncertain about them. While I am not familiar with the Northrup-Grumman plan (although I am an Navy vet who was on the commissioning crew of the USS Theodore Roosevelt (CVN-71)) built at NN Shipbuilding & Drydock Co), it sounds like you are invested in only their stock through this plan. You possibly have other investments in funds offered by the plan as well. All of these investments can be mirrored in an IRA account, should you choose to do so.
To be very clear, taking a distribution from a 401k plan and rolling it into an IRA (or other qualified plan) is very simple. Any company that offers IRA accounts (banks, mutual fund companies like Vanguard, American Century, etc., brokerages like E*Trade or Charles Schwab or insurance companies) will help you through the process if you open an account with them or roll into an existing account.
So, with the limited knowledge I have of your personal situation and risk tolerance, I would recommend that if you don't have an investment ad visor, you simply opt out of the new company stock/plan and take a full distribution of your account. Then you will have bought some time to roll it into an account of your choosing outside of the plan. Here are some rules for the rolling over of a 401k plan:
Below are the options that are available to you.
Make sure you meet all of the requirements of the letter you received.
Based on what you told me, this is all I can offer at this time by way of an answer. I hope this helped! If it did, please let me know.
I am very grateful for the question and I know you will work your way through this.
Best of luck!
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I read the info on your benefits site and it looks like you can't do anything until after the transition takes place.
Once that has happened, all of the normal plan functions will be reestablished and you be able to make elections as to where your funds are allocated.
Based on your initial message you indicate that you own stock through the stock fund. You will automatically receive shares of the new company stock. Only after that happens can you follow the recommendations of my original message.
All of my original comments still apply. If you aren't comfortable owning shares of the new company in your plan, you should roll to either another fund in the plan (if allowed) or out to an IRA.
I imagine many if not most people in this plan have the same questions as you do. You may want to contact the benefits administrator with specific questions related to your particular circumstances.
As a general rule, I am against keeping money in a 401k plan of a former employer. It limits the choices of investments you can make.
Hope that helps. Good luck!