I also modded my 2-342 (actually the equivalent NAPA Belkamp #, but that shouldn't matter)
My technique was similiar to that described above, and is essentially the same used in working with any sort of copper plumbing. If you have never soldered plumbing with a torch before, it would probably be a good idea to get a short bit of pipe and some fittings and practice a bit to get a feel for doing it.
As mentioned earlier, eye protection is extremely important, and also make sure you have removed any flammables from the working area - Murphy's law says that if there is anything that will be set on fire or damaged by a torch within six feet, the torch will set it on fire or damage it!
I put a plumbers union (female side) on one end and on the other I cut off the long tube that comes out of the rad, trimmed most of the bent bits off, and then used a nipple to splice the straight part with the flare to 3/4" back on so that I'd have a straight inlet pipe almost 120mm long (so I can get around a fan)
I used solid core TIN/LEAD solder *NOT* the new higher temp stuff that you are supposed to use on potable water plumbing. (It is now illegal in the US to use lead solder on drinking water plumbing) The Tin/Lead solder has a lower melting point. In addition I used plenty of plumbers acid core solder flux (not electronics rosin flux)
I very carefully flaired and straightened the ends of the tubing coming out of the rad until I had a good snug (not tight) friction fit on my fittings, then used emery cloth to clean and shine all contact surfaces. This is VITAL, the solder won't stick unless the surfaces are totally clean!
Then I filled the rad with cold water to just below the level of the tank / pipe join - this put about 3/4" of space between the water level and the end of the pipe I was soldering on.
Then I did a final cleaning pass on the solder surfaces, and brushed a good layer of flux onto each surface, making sure to cover all of the copper. After I had done this, I slid the fitting over the pipe to it's final placement.
I then carefully heated the junction with a propane torch (I have a hand held torch with a hose that attaches via an adapter to a 20lb BBQ grill tank) until the flux started to smoke, and touched the solder to the junction on the opposite side from the flame. When the joint is hot enough, the solder will melt and wick up into the joint. Immediately remove the flame and gently wipe the excess solder off the joint with a rag or paper towel.
I repeated this with the other fitting on the opposite end and it was done.
Notes...
1. It is important to put the plumbers union together before soldering. This keeps the solder from possibly getting onto the join surfaces, and keeps the female side nut out of the way. After the joint has cooled, dissassemble the union and use the male part on the rest of the project.
2. When I was using the nipple to attach the cuttoff pipe extension, I first soldered the nipple onto the peice of pipe, (using the same techniques except I didn't bother with the water) then fastened the assembly to the radiator. This minimized the amount of heating done to the rad.
Hope this helps...
Gooserider