HemiKiller 'Dennis' posted a good write up on modifying the harness slightly to convert a non-tach car over to a tach car.
there is a bad one on the net that shows you how to cut up your harness to make it work, don't follow that.
the best way would be removing the non-tach harness and replacing it with a tach one. i know that is not a great option since it would cost you a few hundred dollars.
another option, instead of making it look stock behind the dash, you could run a set of wires into the engine compartment, i believe it needs ground and the Distributor side of the coil to make it work.
<-- i think this is the 'bad idea' version.
i know a lot of guys that keep the idiot lights on the left side of the instrument cluster with the clock, and then run a A-piller gauge pod with a tach and oil and vacuum gauges to avoid dealing with the center gauge cluster also.
going the replacement route swapping harnesses can be dangerous to your wallet. I found out that there are at least 7 slightly different harnesses per year for all the different options ford offered. differences exist between sportroof and mach 1 and 71-early 72 cars and later 72 cars and 73s are very different, so you REALLY have to research what car the harness came out of and what options the car had.
here is a good difference that cost me 100$
71 cars included the underdash lighting and map lights on the main harness even if you didn't get them as an option.
early 72 kepted the underdash lighting plugs, but ditched the map light part of the harness and moved it to a daughter harness that was sort of spliced in with plugs. Well i bought a map light for my car, DOH!!!
73 they totally removed the underdash lighting plugs and map light plug if it wasn't ordered with it.
The V4 and V2 harness is different slightly if you had the seatbelt warning lights its different.
you would have to find a car similar to yours from the same year and if possible the same plant that made it to have a really good fit.
That is why a lot of guy either forget about converting it, or go aftermarket.
one thing i don't like about mounting a A-pillar gauge is you drill holes in the A-pillar cover. the A-pillar cover is REALLY rare depending on your body style (sportroofs) and once you cut it; It will cost tons of money if you ever want to restore it back.