i have an old house boat. it has the old 270 hp 350 mercruiser w/outdrive. wanting to put in an rpm guage in the upper station. i found out that it takes a special tach for the old thunderbolt 1 ignition. the tach uses a negetive pulse. all other tachs that i can find use a positive pulse. is there anyone out there that has a suggestion or may have an old tach that would work that they may want to sell?