6V positive ground?
12V negative ground?

They currently do the 12V electronics.
I doubt the market for a 6V positive ground would be large enough for the design and manufacturing costs to be covered, even if you did one that could be used for GM and Ford engines too.
A quick search turned up these:

http://www.jalopyjournal.com/forum/showthread.php?t=258497

http://www.ytmag.com/store/product_files/1142P6_2591.htm