Lower flex when torquing buffer tube

[Superhero]

Hi all, I just finished up my SHTF SBR and I had a couple of questions. I used a bev block in the magwell and the lower receiver (Aero M4E1 Liberty) flexed A LOT when torquing the castle nut to 40#. I stopped cranking when I noticed and used my leg against the buffer tube to provide more support and then torqued it to 40# without any obvious issue. The buffer tube is straight, the lower looks straight and I can't visually detect any cracks or anything...

I know Colt has some proprietary alignment jig they use, but am wondering if that kind of flex is ok. I'm guessing not.

My two questions are:

1. Do I need to be concerned with any damage or weakening of my lower receiver from the flex? I had already installed the trigger etc. as well, could there be issues with the parts or the pin holes in the lower?

2. Is there a better way to do this?

Not only does this lower have my SBR stamp but it's also a limited run from last July 4th with a really pretty "Liberty" shield rather than the usual Aero logo, so I'm very concerned about having damaged or weakened it in a way I lack the expertise to notice.

Thank you!

And yes I searched.