Building or buying an IVR / step down voltage buck converter for the instruments

All I can tell you is search/ google/ etc. There used to be PLENTY of DIY websites on those IVR projects. I could do so, but decided to go with RTE

They are nothing like boost/ buck, however. They are a simple 3 pin linear regulator from the nominal 14V to about 5V

Also if you get into problems with fuel gauge calibration, there's a product called "meter match" as well as a chinese knock off on ebay. The link I had is no longer valid, you will have to look for it.