Fender well headers

download the instruction PDF and it will have a template that you can print out. PDF are not scaled s it will print out to good dimensions. The hole is to install the header as you 'screw' it into this hole and what you are left with is a gaping hole around 3 pipes. Make the hole with a plasma cutter or a body saw and KEEP THE PIECE! use the piece to fab a slightly oversized patch panel that you can do a final cut to fit around the pipes and then pop rivet it or screw into place as youll need to remove the panel to remove the headers. Give yourself enough clearance that the motor will not torque into the panels.
Here is an old vid showing a 65 inner being removed, note the size of the Super Comp hole where the rusted panel is at the rear.