I would do alot of measurements and only tack it in ,then do a mock up on the front sheet metal ,taking tons of measurements to keep everything straight.
After I was positivly sure it was correct ,then I would dissassemble and fully weld it.
That piece is going to set alot of reveals and alignment points in stone ,once you weld it fully.
Alot of measuring and looking can save tons of frustration and time later.......