I have a Norinco model 98 cause it is suppose to be a copy of the Rem. 870 and now I want to put some stuff on mine, like the police Rem. folding stock and a 7 round extension tube. I wonder how far a 8 round extension tube would stick out pass the barrel

WAY out.
The Norinco's barrel and magazine are set up differently than the Remington.

The Norinco has a true 5 round magazine tube, the Remington holds 4.
In order to have a longer mag tube, Norinco moved the barrel support ring farther toward the muzzle.

Since the end cap and support ring are farther forward, even a 7 shot extension will stick past the muzzle quite a bit.

Quoted: WAY out. The Norinco's barrel and magazine are set up differently than the Remington. The Norinco has a true 5 round magazine tube, the Remington holds 4. In order to have a longer mag tube, Norinco moved the barrel support ring farther toward the muzzle. Since the end cap and support ring are farther forward, even a 7 shot extension will stick past the muzzle quite a bit.

Would a 7 shot extension then be a 8 shot extension instead? I wonder how far it would stick out. I have 3 1/2" of barrel pass the barrel support.

Since the Norinco tube holds 5 shells instead of the 870's 4, a "seven shot" extension would also hold one extra.

If you put an extension on a Norinco, you'll have at least 2 1/2" to 3" of extension sticking out.

Another option is to buy a ONE shot extender from Wilson/Scattergun.  This is an extension that adds one shot to a gun.  It won't stick past the muzzle.

One word of warning:  It's smart to use a barrel clamp with an extension to prevent bending or knocking the extension off the gun.
If you give an extension a not to hard bump, the extension can be knocked out of line, or actually knocked off the gun completely, spraying spring and shells WAY down range.

This is BAD, because what happens is, the extension isn't damaged, the GUN is.  Usually when this happens, what gets damaged is the gun's original magazine tube threads.
That means a trip back to the factory to have a new magazine tube brazed in place and for the receiver to be refinished.

This is expensive for a Remington.....it's flat out NOT AVAILABLE for a Norinco.  Damage it, and junk the gun.

Since a standard magazine extension on a Norinco will stick out beyond the muzzle, the risk of hitting the extension is MUCH greater than on a Remington.

For this reason, BE SURE you buy and use a clamp to protect the gun.