Those welds aren't as big as they look, I used the grinder quite a bit to erase my welding mistakes since I was learning to weld on that project! By this I mean that I didn't have to fill any large holes, I just sucked at welding then. I got a lot better by the time I tackled all that pipe welding for my turbo.
I just cut the piping with a die grinder at angles to where the two pieces would butt up together then welded them together. Making a smooth transition was perhaps the hardest part. after they were together I just cut the joined pipe off flush and added about 2" of 2.5" pipe to lengthen it and stick back into the flange. The flang accepts 2.5" ID pipe IF you cut out the old pipe.
war...
