What you are refering to is a base loaded antenna. The device you reference to is know as a loading coil.

Yes you can adjust the 27 mhz loading coil using the proper length whip for a VSWR of 1:1. It will show the perfect match using the swr bridge "meter".

However you still have a poor antenna as a 102 whip will be the most efficent for a mobile antenna at 27Mhz.

The top of any metal bus will work as a groundplane.

If you are interested the American Radio Relay League has many great books on antennas and antenna theory.