I have bought Isspro gauges on several occasions and am currently running four Isspro tachs on my Hatteras for my Cummins diesels. After Cummins and VDO (which, by the way, stands for "very dim organization") couldn't sell us tachometers that worked, I called Isspro and had functioning tachs the first time out within a day or so. Not only that, they arrived with the DIP switches already set AND they agree from upstairs helm to downstairs. I can't say enough good stuff about Isspro- they really know their business. I also have their pyrometer/boost gauges, which of course worked first time out and have continued to do so.
If you have a particular need, ask for Don. I can't remember his last name, but he does their special and custom work. I am going to order the fuel level senders for the tanks in the GT40 I am building from Isspro. They'll fit, they'll work, and the cost will be reasonable.