Pappa,
What gungadoug said is absolutely correct, you need to have a gunsmith look at and fix your trigger problem before going any further. A friend of mine did a 45-70 conversion on an L.C. Smith shotgun for a friend of his. The friend wanted to keep the gun sound to shoot shotgun shells in it also. So the gunsmith turned out a couple of 45-70 rifled blanks to fit inside the shotgun barrels. They were left a little long and threaded so that threaded round nuts could be screwed onto the barrels at the muzzles.

The first problem was how to regulate this beast. He turned out some round eccentric sleeves to go around the reduced barrels at the muzzles, thinking that they could be turned and index the barrels that way. The only problem with that idea was that when they went out to shoot in the gun, the barrels kept twisting due to the rifling in them and throwing everything off. They finally had to weld little ears onto the barrel liners at the breech end and cut matching divots into the shotgun barrels to match the ears so that the liners would not turn around inside the 12 gauge tubes.

They told me later that they did get it regulated, "after a fashion", but I gathered that neither one of them were entirely happy with the outcome of the project. My point is, that if you want your finished gun to be well regulated, you should do the monoblock technique and forget about barrel liners. Monoblocking is neater, quicker, weighs less, and can be regulated easier. The gun will also weigh less when finished.

Even if you reduce the diameter of the liners over most of their length, I am afraid that your gun is going to come out heavy and awkward. Much better and easier to just do it correctly from the start.
Bob