This has been bugging me for a while now – I have been wanting to trim the top lip off of my new patch piece, but have been struggling with how much I could trim.  Placing it in the car was problematic as the lip was preventing it from being flush and kept it raised and wouldn’t sit in there straight:

Then the light went on and I placed the piece that I had cut out of the car and used that as a reference:

I will be able to trim that lip off and have room to spare!  So after I trim the lip, I will position the new piece and trim the opening in the car around what is left.  Then I will be able to butt-weld it in place and can start on the other side.  I will also be able to place the firewall bracket to use as a rough guide as well!