my radiator panel has rotted out at the bottom like everyone else's.
I have bought the replacement panel.
Originally, i was thinking i would do careful measurements, cut both in exactly the right place (somewhere near the bottom of the vertical strips) and then butt weld them.
However:
1) my welding skills are not great, so i am not sure how good the result would be
2) making the cuts at precisely the right place with an angle grinder would not be easy - i would have to do my best to allow for the width of the cut, and i suspect it would still be difficult to get this right to the correct mm.

So i was wondering about buying one of the little air flange tools and putting a flange in the replacement panel, so that the flange sits behind the existing panel. This would
1) make the welds easier and stronger - i can weld on both front and back, and possibly use filler on any uneven sections
2) it makes measurement a bit easier - i can position the overlap so that the result is exactly right.

Has anyone else done something similar, or is there a good reason why i should not use the flange tool?

I think you will find the flange tools will struggle with the thickness of the material

Measure and cut with a saw, that's what I did. Think the flange making tool is only for thin modern car panels, you can buy hand operated tool.

I have a hand flanging tool, works with effort on car panels of 0.8mm -0.9mm but suspect it would struggle with the rad panel.
Mine is sitting awaiting the same treatment having rotted out at the bottom, will watch yours with interest.
However it you are going to flange why don't you just cut a strip and place it behind the join, plug weld them into the correct positions and then weld the join?

I repaired mine with some thicker steel that already had a bend in it that matched the radius at the bottom, had a similar dilemma but mine did not need to go right to the end and all you really need it to sketch how to do it and take a few measurements from the fixed points ie the base or the openings and ensure they stay the same, im not a welder but practiced and made a reasoanble job and that was 14years ago, . i think experienced welders leave a small gap to fill in with weld and i found short spots or stitches worked well, and when grinded down and filled looked good, dont under estimate your skills, if your'e butting onto good metal it should be fine

Made this from folded sheet from metal shop...

Don't know if this would be of interest. I made this high tech gadget to hold everything in the right place. It meant the repaired panel must end up the same shape in the end. Probably total overkill but that's the way I roll baby!

Oh and I got the repair panel from Tony at The Welding Wood and sent back the Britpart one as it was bloody awful.