Cut off the uneven edges of the old pan, make it one flat lip. Cut a single piece that comes 90* off the old pan into the pieces you fabbed. No messing with working-in odd shaped small pieces. You could even run the single piece at a 45* angle as opposed to 90* for a smoother transition. And I mean one piece cut per side. The front portion of the pan looks like it is just about touching your fabricated piece to where you could just weld it.