Andreas,
My best advice to you would be to concern yourself with the horizontal spread first, before you worry about getting them together up and down. After you get them spaced the way you want them, then you can adjust them for up and down. You may also find that as you bring them together on the same horizontal plain, that they will spread slightly apart more. This is just natural and is a reaction to the way they are situated next to each other. I use the monoblock method and temporary spacers to get them on roughly on before I solder anything up. Some of my best groups were shot using aluminum temporary spacers and hose clamps!
In order to do it this way you need to make up some temporary bolt-on sights. The whole mess looks like hell while you are doing this. Friends have had great laughs at my expense when I show up to do preliminary sight-ins with the gun rigged up this way. But they usually shut up when they see the results. I shoot most of my shots sitting down at a bench but I hold the forend the same way as I would when hunting. I rest my forarm on a sandbag but let the rifle freely recoil just like it would if I were standing up.
Everything you do to the gun or it's barrels will effect the way it shoots! I once regulated a rifle with the old shotgun wood on it, which included the original splinter forend. I got it shooting great, then I restocked it along with a new beavertail forend. That was enough to throw the regulation WAY off! It limited the rotation around the center of gravity a lot more.
I usually shoot two rights and two lefts, then open the gun and let it cool while I go down and mark the target. I have used two targets before, one for the right barrel and one for the left, but I found out I like to have them all on the same target as a referrence so that I can refer back to it when I do the adjusting. You need someone spotting for you when you do this and either marking them as you shoot or at least mentally marking them so you know exactly where they are going.
I hope you were able to buy more powder, you need to shoot it more and move the barrels less. I never correct any soldered joint till I am certain it needs to be moved, otherwise you will just be moving it back and forth all the time. Having a scope mounted will help your regulation shooting, but don't be surprised if it changes points of impact when you remove the scope. Bob H.