Wow! Very impressive! I thought I had a few tricks up my sleeve on these old cars, but after reading that, I’m grabbing my bag of zip ties and pop rivets (not to mention the JB weld and tube of weatherstrip adhesive) and getting out of the way.I recently installed and 2019 6.0l with a 4l90 into a 63 Cadillac model 62 convertible and couldn't find patch panels either. The trans tunnel also had to be significantly enlarged to fit the new trans since it was twice the size of the old one. I ended up having to buy a sheet of 18ga steel and make my own piece at a time. I had to buy a bead roller and a bench top shear to do it but and make multiple smaller panels to weld together but it worked and came out nice. Stronger than original but it was time consuming. You will also need to invest in some Cleco pins and I reccommend TIG welding it vs MiG as the mig heats things up to much and moves them around.