Here's a tool that I use occasionally, but have found to be invaluable. It's pretty simple to build, and will make alignment of many round pipe or square tubes a breeze. I didn't invent this, but learned it from a farm welder.
Start with some scrap angle steel, and cut two pieces of equal length. Mine are about 6 - 8 inches, but could be longer depending on the size of pipe/angle used. Then weld them to a "C" clamp.
...you can do this easily by placing the angle onto a uncut section of pipe, then welding on the "C" clamp. I used a low cost C-clamp purchased from the "China store", since it was at hand and wouldn't cost me much if I messed this up... Here you see I've clamped it to the leg of the stand for my metal cutoff saw:
Here, I've photo'd it clamped to the leg of a table. I used this very clamp, to extend the legs of a table, that I salvaged from the scrap yard. The pipe used to extend the legs, was also from the scrap yard...
I hope folks here find this as useful as I have.