I have a custom built AR10 and I have a Nightforce NSX in Spher mount a top. When the bubble is in the center of the mount and "the scope and mount were level when I purchased them together" it seems as if the scope is crooked. Is it possible to have a bent upper receiver.

Loosen the rings, straighten the scope, re-tighten. I highly doubt your receiver is bent unless its been severly damaged. Leveling a scope is pretty simple.

I have a hard time understanding what you are saying, but I suspect wareagle is correct, its as simple as loosen rings, correct scope, snug rings.

leveling a table and using a plum bob will work but can I trust the level built in to the mount or should I put on a separate level on the scope?

I seriously dough the receiver is bent

Given the fact that its a Spuhr, I would trust the level thats built in. However, all the scope levels I own are currently at the bottom of a parts bin.

Don't over think it, just shoot.

I shot out to 1000 quite often, AR and irons... and to 600 same almost every weekend if not twice each weekend.

I have never owned a level.

I don't even own a level on my scoped bolt guns...

Now of course what you might be doing with the gun could change my mind but generally speaking I've never seen a need for them.

I would not understand a level mount that was fixed. Makes no sense to me because your shooting position might be best with a bit of cant, and you'd want to zero that out in the bubble..