Custom Shotgun

What do you mean exactly? Do you mean, what shotgun would I pick and how would I fix it up? I'd take an 870 and turn it into a Scattergun Technologies clone. Good Shooting!

Which Scattergun Technologies model ?, the K-9 ?, Border Patrol ?...I've got an 870 Marine Magnum that I want to have ghost ring sights and a full choke installed

Oops the K-9 is based on an 11/87...

I was referring to the 870 that is used in all of the Wilson Combat advertising.

sounds like the "Border Patrol"

You can check them out at
http://www.wilsoncombat.com/sgt/sgtindex.htm

Is part of the '94 gun control act effect shotguns? Does the extended mag on shotguns fall into the 'over 10 round' mag category?

I'm not sure because the tubular magazine is fixed..

It does count as a magazine with 10rd limit. The ban only exempts tubular devices designed only for 22 rimfire, such as the Marlin, Winchester, etc.
And semi-auto shotguns are affected by the ban in this way-
They can have only one out of four features:
1. Fixed mag over 5rd
2. Detachable mag
3. Pistol grip
4. Folding stock
Some of those are the same as rifles. But flash hiders, bayonet lugs, and grenade launchers are unrestricted on shotguns.