I'll bet your bodywork wouldn't be as dodgy as mine, that's why I don't do anything important myself!!
As you grind away at the top surface of the weld, it will come apart/loose on it's own, so only need to go deep enough till this happens.
That's why the correct bit is so much better as it centres and pilots all in one, then has the cutter bit as a ring or sharp teeth around the outer edge, saves loads of time/hassle .
When wing is off, grind smooth where it was fixed so nice and flat, drill some 10mm holes through the lip of the new wing so that you have some adjustment where you can wiggle it around, get it where you want do up self tapper screws to hold in position whilst you spot, file/grind away excess, take your time and you could get a decent job, pay particular attention to the panel gaps, and rust proof as you go along to make things last.
I used to live in Colchester, at IMPHAL CLOSE, no 10!!
tabetha