Tach in '77 reading roughly double rpm, HELP!!!

[IDIDNTDOIT]

I restored a '77 3/4 ton 4X4 suburban (one of only three I have seen with factory 4-spd). I put in a factory tach from a '73. I didn't know to get the filter on the distributer.... A friend, that is a GM wrench, said I needed one but they are obsolete from GM so I orderd one from Classic Gauges. My tach reads roughly double the rpm's the engine is turning, I tried it with the filter, w/o the filter, tried reversing the wires.... I don't know what else to do????
This is driving me crazy, and It also scares the hell out of any one that drives it!!!

Thanks for the help,

Bill