Weldox,
This is a long post, but here goes. My first recommendation is just go with the post-ban setup you already have. Front sight bases are located and then drilled and taper reamed on each individual barrel. No two are the same unless they are done on the same equipment and even then it may not be exact.I have a clamp-on AR15 front sight base from Armalite. They work very well. Has two socket head cross bolts. Locks up solidly and you can even use some red high strength loctite (make sure it is where you want it because this stuff takes about 400 degrees f. to loosen) to really fasten it down. Squirt some compressed air through the gas tube to be sure there is no locktite in the gas port. Looks just like the factory sight and (for U.S. shooters)is approved for CMP competition. However, it does not have a bayonet lug. If you get another pre-ban factory sight, it will need to be located and drilled. Roll pins are not as good as taper pins. I've had both kinds. Another person suggested using set screws. Since the new barrel is already drilled, the set screws will go into the cross drilled holes and not do anything positive for windage adjustment, so forget that. To use set screws properly, there must be 4 small flats milled into the barrel for the screws to ride in (on undrilled barrel). When drilling and tapping a sight base for the screws, the holes need to be located slightly higher up than where the taper pins go. This allows full bearing of the screws in the milled flats. Properly set up, the set screw sight is great for a target rifle. Your new barrel is already cross drilled, so your gunsmith can use a new undrilled front sight, or the old one if the holes are really close, but you already said they did not match. He will most likely have to go a tad oversize with the drill and tapered reamer to clean up the existing holes, but taper pins come in many sizes and are readily available. Tangeant described the way to index the front sight for windage prior to drilling. Tap the hole below the sling swivel for a temporary set screw, center the rear sight for windage, install front and shoot to confirm windage, then locktite the front before drilling and installing crosspins. I don't like them, but Bushmaster makes a bayo lug that clamps around the barrel in front of the sight base.