Art, KB9MZ wrote:
"---it should provide good local contacts if placed at a height of 70
feet."

If back to back mobile antennas are used as a dipole at 70 feet, the
question is: why not use the vertical distance for a radiating
conductor, and the horizontal space to load the too-short vertical?

1/4-wave at 160-meters is about 131 feet.

A vertical mobile antenna is likely limited to about 4 meters and a
dipole made from two is only about 8 meters long. That`s only 10% of
1/2-wavelength. Radiation resistance of such a short dipole will be very
low and capacitive reactance will be very high requiring much reactance
to tune out. This is inherently lossy.

Another disadvantage if the dipole is only about 1/8-wavelength high and
horizontal, will be concentration of most radiation directly overhead.
For local and DX coverage, energy concentration near the horizon is
important. Vertical antennas provide low angle radiation even when they
are short. They have a null directly overhead.

70 feet is 53% of the height required for 1/4-wave resonance at 160
meters, and some of the missing length can be made up by horizontal
loading at the top of the vertical height.

ON4UN`s "Low Band DXing" Chapter 9, "Vertical Antennas" can be very
helpful in choosing a configuration that will work well.

Best regards, Richard Harrison, KB5WZI