What has to be done to a motor to accept NOS?

Can you use a good block?
Normal Crank?
Pistons and Rings?
Camshaft?

Seems some of the people, like on pinks, just have normal parts...

Well, nothing HAS to be done, but if you want the engine to live a lot longer, there's a few things you SHOULD do.....

A 4 bolt block or 2 bolt with splayed caps making it a 4 bolt is a really idea, a 2 bolt small block wouldn't last very long.

Forged Crank is recommended

Forged Pistons are pretty much standard, unless you like melting them, or shattering hypers

ARP waveloc rod bolts(also ARP main bolts or studs are a good idea) or comparable

No higher compression than around 10.5:1....just like a supercharger increases compression, so does nitrous so that's the reason for the limit of compression points.

Gap your rings a couple thousandths more for the extra expansion

A larger duration camshaft will forgive some of the extra cylinder pressure experienced while on the juice.

Also, a rule of thumb is to retard the timing 2 degrees for every 100HP shot, so if you're using a 200 shot, retard it 4 degrees, etc.(with a timing light of course)

The rule for timing is 2 degrees per 50hp unless you are using real good gas like C16 or something along those lines.

If you are talking about a street car that is a whole different ball game than a race car. The biggest thing is forged pistons, while cast pistons can handle a fair amount of nitrous when tuned properly there really is no room for error. As far as blocks, cranks and rods go the same rule applies with nitrous as with anything else, if your overall power is going to be more than a stock crank, or 2 bolt block can take you need to reconsider your choice in parts. When you get to large amounts of nitrous then you will start to see pistons made with a lower top ring and other things to increase strength and longevity, but this is beyond what most street set ups need. As far as compression is concerned nitrous does not increase your dynamic compression like a blower or turbo you will however get a lot more cylinder pressure just due to the increase in power you are making. The upper limit is going to be probably around 15:1 although most guys running multiple stages are running around 13:1. For a street application I wouldn't be too worried about the compression just aim for a compression ratio that is doable on what ever gas you are trying to run. You can run up to about 150-175 on pump gas assuming the engine currently runs well on said gas. After that you want to start thinking about running some higher octane fuel. Since for a street car the nitrous is used for such a small part of the over all driving I would build the engine to do what you want it to do 90-95% of the time.

well i have a 454 2 bolt block.

I am going to buy a forged crank.

Pistons are TRW L2349F domed.

I have stock car rods so I am thinking I might have a problem there...

049 heads stock

Weiand 8001 (Action Plus)

Dont have a cam yet and dont have a carb.

It is for the street but was thinking a small shot of NOS might be a little bit more impressive then the 420 HP...

Dont want to destroy anything though...

You are fine go get some thing like a Super Power Shot or equvilant jet it for 75-100HP to start and have fun. You want to retard timing from where the engine makes peak power and on pump gas I would start with 2 per 50. Once you get it close to dialed in you can add a little bit of fuel to it and add a degree of timing and see if the MPH picks up to get it dialed in the rest of the way.