Hi,
When I built my first double, I had no clue on how to go about regulating the barrels and at that time there was no internet to search.
Here is how I did it: After soldering the barrels to the monoblock I soldered the quarter rib in place. The rib was machined to accept a set of Ruger scope mounts. I then soldered a "figure eight" contraption to the muzzle of one of the barrel. The other barrel went through the other hole, which had three screws. These screws allowed me to move the barrel until I got it close enough at 100 yards.
Today, the next step would get me arrested or at the very least banned from the shooting range. I then loaded my oxi-acetylene set in the trunk of my car and backed it up to the shooting line at the Fish Canyon Range, not far from my shop in Azusa, CA.
I used a 4 X scope. Then, I silver soldered three or four spacers starting from shortly after the quarter rib. I kept a wet rag over the rib. After each spacer, I shot it again and readjusted. The final spacer was just behind the adjusting device. Then I removed the adjuster and shot it again hoping that nothing changed. I took back to the shop and soldered the ribs and front ramp. That's the method that I used.
I did all the shooting from a sand bag bench rest. For the .458WM, I put a 25lbs. bag of shot betwen my shoulder and the recoil pad.

Here are a couple of pictures of the one that I'm making now:

Step 1

Step 23

I will post more as it comes along.