A good weld is the best for combating leaks, but a pourous weld can be frustrating in trying to figure out exactly where the leak is. A major leak will be easy to find, but a small one is a bit more difficult and still sounds bad. If I were you I would get some small pieces from the auto parts store and practice on them to see if you are comforatable with welding the pipes. Generally you would tack everything in place on the car, then either remove if you can, or at least drop it down so you can weld the top of the pipes. The Flowmaster kit uses a sloted pipe (to allow for a smaller ID/OD difference betwee the 2 pipes)and a narrow band clamp. That clamp system it much better than the old style saddle clamp. The old style, as you tightend the clamp you put a depression in the tube. The trick was to get tight enough to make sure the pipes don't come apart, but not so much that it distorts the metal enough to create leaks. The band clamps squeeze around the entire diameter (more or less) of the tube. 3" wide band clamps are available which seal even better than the 1" wide.

One thing to keep in mind though is serviceability. If you weld it in solid, it's very difficult to remove. If you have to do trans work or other issues under the body, it may be in your way. Myself, my system will be in and out a couple of times since I am still in process of restoring/modifying my car. For the present time, I will weld the front pipes to the h-pipe, but then clamp it to the muffler so I can drop the entire h-pipe when I do my trans swap. When I am all said and done, I will probably weld a serviceable flange to the h-pipe at the muffler and weld the muffler solid to the tailpipe.

That's a decision you have to make based on where you are with your car's progress and what you anticipate for accesability in the future.