Thanks for the memories Greg. In the 60's we would do underbody mounts of the antennas when we were stripping chrome, filling holes and "frenching" headlights. Don't remember ever having a problem with reception. There were/are some special antennas out there. Take a look at this option:
https://www.crutchfield.com/p_12044U...l-Antenna.html
Enjoy the adventure, Chap