F/V and V/F works and is analog. The projects mentioned actually used it for fractional changes to the speedometer caused by changing tire sizes.
In your case, I think digital will be easier, as the ratio is exact. Converting sine to square is not a problem.
At 1200 rpm, your 4-banger puts out 4800 pulses per minute (i.e., 4 per revolution). That is 4800/60 = 80 Hz. Your 6 cylinder puts out 60 Hz. So, 80/60 = 1 1/3 or 4/3.
I have to run now and won't be back for about 5 or 6 hours.
John