How much nos

How much nos can a stock block a460 head handle.
The engine makes 909 hp@ 7000 rpm na.
Block has a 4 bolt main conversion
And filled with concrete
All forged rotating assembly.

Well, the answer is rather vague. No one knows what it will hold before it lets go. Some blocks seem to be indestructible, and others will throw everything they have, under your vehicle!
I am sure the mods you have done to it will help it achieve a higher threshold compared to not having those mods, but no one can really say where that threshold is!

What block is it?....a D0VE-A block? a factory Boss 429?, or a CJ block?

At 900 hp in NA form, I would think that you are already close to wherever that threshold may be. And the introduction of Nitrous, depending on how much, and how "hard" of a hit it is, just may find it right away!
But you simply dont know until it happens.

Thanks for the info
The block is a 1973 460 block
Was thinking about a 200 shot

all in the tune, start out smaller , and read the plugs !!

I spray 200 on a .060 DOVE A block with factory crank and main caps/bolts but I'm not at 900 either , but then again its got two bolt caps and not filled , best so far was 5.82 at 120mph with a 1.26 60' , i was trying to hit high 70's but i hit the rev limiter pretty good because i didn't grab 2nd fast enough , that 2nd gear comes up quick in the C6 , I footbrake at 2500 and shift at 6800. Its a compucar plate with 78n-52f jets and a hp950 carb jetted 78-88 with a front pv.

I was hitting my old motor with a 98n/80f jet on a NOS plate. Motor was a little 488 using diamond pistons, BME aluminum rods, and STOCK offset ground crank with TFS A heads. Block was a D1ve 2 bolt block that was filled to just below the water pump holes. Motor made 875hp before the hit, and all was good, but then again I stayed on top of my tunes after blowing up the 466 from a bad tune up.

All of it?

Is this the new TFS A heads or the old Motorsport A heads? The TFS heads have a thicker deck I believe.

I would think the block would take a pretty good bit since it’s filled. But then again like the other guys said the tune up will be the key.

What bore & stroke?

One other thing I didn't see anyone mention, your nitrous controller! Your nitrous should give you a "push" rather than a "slam". Using a controller, phase in your 200hp shot over the course of 1-1.4 seconds. Use a good MSD 6AL or 7 to automatically retard your timing when your nitrous is turned on. Your phasing of the nitrous should start at 25% and reach 100% after 1-1.4 seconds. This will keep you from scattering engine parts all over the track. The "Fast and Furious" sceenes where they slam nitrous in and have to grab the steering wheel to keep from ending up in the back seat is fine for the movies, but it's terrible for your engine. Ramp it in and save your motor instead. With the build you described, a 200 shot is a definitely attainable goal. Use your box to retard the timing. I believe the recognized standard is 2 degrees/100 horsepower. Sounds like you got a screamer!

I misspoke on the retard numbers. It's 4 degrees per 100 horsepower. Sorry.....