1977 c10 Factory Tach

[modela1929]

I have a 1977 c10 with a 350 engine and it has an MSD Street Fire Distributor 8365 I put an after market tach in and it works fine I since found a tach dash installed it and all works fine except the tach reads nearly double the actual rpm Is the problem the used tach or does it need a different signal from the distributor I called MSD They said it needs a 10k ohm resistor in line another said it needs a tach adaptor box #8910 the third guy said my tach is the problem
I did read some where that when the factory tach start to fail they read high?
Can anyone help me
Thanks