I have dual Carter-Webers on my 6v truck and the chokes work perfectly. Initially, I was also worried about a 6v/12v problem. Maybe the choke opens slower but it is fully open within a minute.

Here is an earlier thread with a part number for a oil pressure switch that works.

This thread discusses the fittings. NAPA has changed their website and that link is no longer good - the part numbers are listed, I think.

Here is the NAPA switch.

Here are tee-fittings that might be needed.
Here is a tee for 3/8" pipe threads.