The only tach option I have ever seen is a sun tach, I bought one and had it retrofitted electronically so it would work with my tbi 350........scored a new old stock batometer also, again this is a factory option for a 1/2ton gmc per jollys website. I would not count on a mechanical diesel tach being driven from a 305v6 easily.....???
http://kieth.smugmug.com/Trucks/66-GMC/i-X8WmmMg/A