I stumbled across a website called 39-47 Dodge, for dodge and fargo trucks and there was a thread about www.westach.com, said they make brand new 6 volt positive ground tachs. And they do!
Go to the site
left side it will say TOOLS
Click on NEW build an instrument
It starts with gauge size 2"-3 1/8"
Then no. of instruments-like single or cluster
Type of instrument next-Tachometer
There is a whole page of tach's, not all 6 v pos ground, from 6,000 rpms to 8,000 rpms. Even tachs at 3,500 and less! Some say Std Pt Ign. that is what I was looking for. Happy hunting!