I did exactly what Jon said last night, but to a pair of Unimog rims, exactly the same construction.
I'd already done a couple, so I knew the technique.
One took me 20 mins to grind and beat out the centre
The second one took me nearly an hour
If you grind away the bulk of the weld with a grinding disk, so there is a right angle between the rim and the centre, rather than a pool of weld.
Then get a thin (1mm) cutting disk and cut into the corners to get to the hairline crack to appear.
Don't forget to grab a cold beer when you are done.
Welding mine up will keep me busy tonight.