Shaping & manipulating both left & right subrail extensions pieces to fit took a long time but worth it. I'm happy with the fit both against the quarter and over the chassis & crossmember. Every time I shaped the edge the base would alter too so it was a challenge.
Brookville rear left subrail extension with quarter panel held up against it in situ. It was a lot of tapping & shaping to get that little piece to fit better. The curves were easily 1/4 inch from correct quarter panel profile.
All trim cuts, edges & inner cuts were first measured several times to be sure. I like to leave a bit extra to then trim for a perfect fit, especially panels which originated from USA. Each hole was also double checked before drilling to avoid errors. So far so good!
Brookville floor panel trimmed down to fit with rectangular hole cut out for rear Model A Ford crossmember. Approx 1/2 to 3/4 inch was trimmed off sides to achieve a neat fit. I also rounded all corners, inner & outer.