Hunta,
I would rethink this project if I were you. I'm not saying that you can't convert your shotgun to some kind of black powder rifle but I'm dead sure that the 500 express is too large for the purpose. I would limit my choices to something on the order of a 45-70, 45-90, 50-70, or 50-90 if you need more power. I would also forgo any threading of the monoblock because with it being made out of damascus material, you don't don't what kind of problems you will run into trying to cut threads in it. Leave your monoblock at least three inches long, then ream it to a straight walled even diameter and solder your barrels into it with Brownells Hi Force 44 solder.
If you have never soldered steel pieces together before, send me a P.M. and I'll go over some methods with you that will allow you to get nearly 100% coverage/adhesion. That is important to the strength of the joint. Also, you need to fit the barrel stubs to the monoblock with only 5 to 6 thousanths of space between them, this will leave enough room for the solder to flow into the joints well and ensure that you get enough of it in the joints.
Make the barrel stubs an easy slip fit and leave them about .010" longer than the monoblock, this will give you enough length on the stubs coming through the monoblock to file and polish them down to the proper length. When I bore out a monoblock on my lathe, I always leave the barrels cut to about 10 to 12 inches long. This allows me to make sure that I have the barrels lined up from end to end on centers and ensures that the new monoblock will be bored and reamed exactly along the center lines of the old barrels. This will give you a bit of built in convergence to your barrels and that is desireable. You will need to reset your holding fixture between cutting barrel one and barrel two because they are not parallel to each other.
I hope this helps you out, if you have any further questions send me a P.M. and I will do my best to help you out. Good luck with your project gun if you decide to proceed with it. Bob