Cuda 5.7 //////harness to Rally dash tach

Bought a running 73 Cuda with a Gen 3 5.7 conversion. It has an interface done on all the other gauges in the stock rally dash. Was told to buy a MSD 8910 tach conversion module. Anyone wired up stock tach to gen 3 harness? Looks like I've got a violet wire in the 5.7 harness that sends a tach signal. Is the 8910 the correct conversion piece? Can anyone help with what goes where to make it work.

I am assuming this converts can-buss to conventional? Would need to do the same with the tach if that's what the msd does.