The book says to cut out the section of BR1 after welding, I wouldn't do that i I was you. Much better and safer to cut this out prior to welding as it's this section that will lead to the tunnel section being twisted.
Better still is to cut the tubes that form the tunnel opening with 45 degree angles so you don't have any open ends....doing it this way you could fully weld and dress this up before tacking it into the chassis...simples