This conversions worked on my 5.0L with MSD, should also work on yours:

The stock Volvo four-cylinder gasoline tach can be modified to work on the V8. If your tach is made by Yazaki (printed on the tach circuit board edge) and it was used on a gas engine Volvo, you can alter it to read for 8 cyls. yourself The diesel Yazaki tach will have to be replaced by the gas tach.

Parts & Tools:
-1/4 watt, 100k ohm resistors
- pencil soldering gun

1. Remove the tach.

2.Holding the tach with its face towards you, look on the right, top side of the printed circuit board, approx. under the highest RPM. reading for the resistor (has color bands) that has "RV" (resistance variable), printed under it.

3.On the underside of the p.c. board, find the where the RV leads come through and are soldered.

4.Solder one (100k ohm) resistor in parallel to the RV leads. You're done.

Depending on your ignition system the tach signal may also need to me modified. On my ignition using an MSD Billet Distributor and MSD6AL I needed to is the MSD Tach Adapter to convert the signal,
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Volvord 784VC www.telusplanet.net/public/gilesij/Volvord