In my opinion, your assessment of the technique for the 4" pipe is correct.
Assuming you have access to the inside of the 2' pipe, I used to use a spring loaded tube (such as an old spring light pole or pogo stick) on the inside of a pipe or tank. Alternately, for only 2' diameter, you could wedge a 2x4 or pipe cut to inside diameter length in that short distance. Place slightly above or below the weld in interest, whichever is more convienient to placing the source. Duct tape (wonderful stuff) the source tube to the center of the tank diameter so it points up or down, to the center of the tank in line with the weld. I assume 12" SFD is sufficient for thickness of pipe. Film goes on outside of tank.
If you do not have access to inside, you will have to shoot from the outside, through 2 walls. Again, film on outside with source 180° around pipe. I assume SFD will be adequate for 2' diameter to wall thickness ratio.
Get technique right at ground level then shoot up in the air. I would assume whoever you hire to perform RT will know several tricks to RTing in awkwards positions.
Good luck!