I gave up on the pipe that I was trying to weld myself...
I started a new one, this time I've only tacked together and it will professionally welded!
Here is how its come up so far :
This is about as far as I can get without jacking the car up and cutting into the existing pipe.
That will be the moment of truth I will see how accurate my measurements are.
However I need one more mandrel bend to complete that bit, so I'll have this piece welded, see how it comes up, check the fit, smooth out some of the welds...while the bend arrives in the mail.