If the 6 volt convertor is rated for the load you intend to impose on it AND you have appropriate overcurrent protection for the wiring (and possibly the equipment) then it would appear that nothing else is needed.
A few questions that might be relevant:
1. A restatement of the above, the obvious, but I thought it best to restate it.
2. Can the convertor handle and suppress voltage spikes that are common to autos?? Will it pass those spikes on thru to your expensive circuit board? Maybe extra protection for this is required.
3. If the voltage regulator fails on the auto the system voltage can go extremely high. I've been told that auto systems designers allow for as much as 60 volts. This could occur if battery was removed and regulator failed to 100% output - at least on some autos. If that happens - will your convertor pass that on, possibly ruining your expensive circuit board??
4. Is the wiring adequate for the current source?? If connecting directly to the battery make sure you have a fuse or fusible link or similar to prevent a fire. If connected to cirgarette lighter - can the wiring handle the max current of the lighter socket?
There are some convertors that are modest in cost and that will do the job for general purpose use. What you have may or may not be suitable and I've shared a few of the things I'd consider if I were powering something relatively valuable.
FYI, except in special cases we reply to the forum, allowing others to share, add or even correct errors or misunderstandings.