Plate was part of underside reinforcements Hemi cars got. Goes with the boxes that get welded between rocker and frame rails in front and rear. Also known as torque boxes.

68 Road Runner (383/4speed, post car w/decor pkg) - Major Project
69 Road Runner w/472 Hemi & 4 speed.
70 Challenger R/T SE EF8 w/ V9J, U - A32 - Major Project
2023 Ford Mach 1