It takes a fair bit of time to get them really square to the head inside butt.
I make up stainless 4 mm high rings with the flange.
And a small bit, interfer fit, into the pipe.
Than hit the pipes into correct position.
Juggle until satisfied. If not a beer helps great here, than take carfully out the next day.
Tag weld, refit with a coat of blue and juggle again, best with a beer than see if it has thrust arround.
Very happy easy to take out and fix pipes afterwards.
Mind you it can take a day