...Mike, I'm guessing that your problem is just that you need to adjust the Tach to the "V10" mode...probably just a tweak of the trim pot. You could try this on your Fiat piece and avoid one step under the dash...
Here is a schem taken from the reliable internet for the Veglia, it is simple. the only place that things go 'wrong' when mixed up on the ignition source is the filtering section there the south-west quadrant. Otherwise, the Q1 transistor pulses vaguely with every spark and pulls a current thru the meter, with a final gain set by the trim pot (R10). I think its possible that All veglia car tachs (no matter 4,6 or 9 cylinder) just twist the trim pot.
The Dinoplex circuit is simpler than it looks, the part internally has its own schmitt trigger to clean up the noise and react to the edge (not just the 'weight' of the spark, as the Veglia) and send a single pulse. (and something I'd be likely to do, the pin names on the CD14538 are a bit messed up...and come on, man, CMOS inputs should never be left floating... 
. But yup, its about $8 worth of parts....Lee
