V-8 to V-6 Tack conversion

[bruce plecan]

At one time I thought I had seen (in print somewhere) of a simple R/C
circuit for doing this (may have been V-6 to V-8).  Just wondering if
someone here has done this.
  While more complicated, would using a programable divide by 10 device
set to count to four, and reset, but just use three pulses to the tach?.
ER, use the three pulses to trigger a 4066, and run like a MOSFET off
that.  I'm skill limited in circuit design, and execution, or would a
optoisolator be better?.
  The reason I ask is that I'm going to a V-6 MPFI, from my V-8, and
would perfer not to but a new tack, and just use the original one.
  TIA  Bruce